A Second Paper on Hygrometry. By J. A. De Luc, Esq. F. R. S.

XXIII. A Second Paper on Hygrometry. By J. A. De Luc, Esq. F. R. S. Read July 14, 1791. PART II. In the first part of this Paper* I have treated of the fundamental principles of hygrometry, and of some hygroscopic phænomena; and this will relate to a particular application of those premises. 62. Since the publication of my first hygrometer, many others have been invented, two of which are now principally in use; the hair hygrometer of M. de Saussure, and my hygrometer made of a slip of whalebone. If the comparative points of those instruments could be determined in the whole extent of their scales, the only inconvenience of their being both used would be, the necessity of reducing to one of them, the observations made with the other; but from 70 to 100 of mine, which space includes the most important period of moisture, their correspondent indications are as different from one another, and as variable, as if they were the effects of two very different causes. Therefore it is important to decide which of them should remain our only measure of moisture, till, if possible, a better one is found. The following pages, I hope, will lead to that decision. * See Page 1. of this Volume. 63. The fundamental process of M. de Saussure, with the view of discovering the effects of moisture on the hair hygrometer, was this. He repeatedly caused successive known quantities of water to evaporate into a close glass vessel, previously reduced to extreme dryness, and containing that hygrometer and a manometer; he observed the correspondent changes of those instruments, and, by combining the results of his experiments, he reduced to regular series the correspondent motions of the two instruments by equal quantities of evaporated water. Having confined himself to that only class of experiments, which, from causes that I shall explain, could not discover to him the difficulties of his attempt, he thought himself warranted to draw from them the following conclusions. 1st, That the degrees of moisture in the inclosed medium, were nearly proportional to the quantities of water evaporated in the vessel; and that, consequently, the ratio observed between those quantities and the march of his hygrometer, could be considered as giving immediately the march of the instrument correspondent to moisture itself; which, according to our common opinion, is a certain quantity of aqueous vapours spread in the medium. 2dly, That when no more water could evaporate in the vessel, the inclosed medium was arrived at extreme moisture; and that, consequently, the point indicated at that time on his hygrometer, was to be the limit of its scale on that side. 3dly, That having, from those experiments, a probable determination of the expansions of the hair by successive equal quantities of moisture, in beginning from the point were this is null, and ending at its extreme, his instrument could not differ essentially from an absolute hygrometer. 64. These conclusions were very natural in the state of M. de Saussure's experiments; but before their publication I had gone gone over a great field of hygroscopic phenomena, in which the hair, and a close vessel, had a share; and thereby, seeing the objects in another light than M. de Saussure, I doubted of his conclusions, and I procured three of his hygrometers, in order to examine them on some particular points. It was after that immediate verification of my conjectures concerning his instrument, that I settled the following conclusions, very different from those above. 1st, That moisture, or the quantity of vapour spread in the medium itself, does not increase in an inclosed space in proportion to the quantity of water evaporated in it; because of an increasing, but undetermined, part of that water being deposited on the sides of the vessel; and that, consequently, Mr. de Saussure's experiments could not afford the determination of a real hygroscopic-scale. 2dly, That the circumstance considered by him as a sure sign of extreme moisture existing in the inclosed medium, namely, the maximum of evaporation in the space, has only that effect when the temperature is very little above $32^\circ$; but that, by successive increases of heat from that point, moisture recedes farther and farther from its extreme; or from the point where no more vapour can be introduced in the medium without an immediate precipitation; though at the same time, there are successive increases in the quantity of vapour, and thereby a constant maximum of evaporation correspondent with the actual temperature. 3dly, That, in approaching to extreme moisture, the hair hygrometer becomes stationary, and afterwards a little retrograde, in which march the unavoidable irregularities of every hygroscopic substance produce frequent anomalies; from which cause it was very difficult for M. de Saussure, considering the form of his experiments, to discover the hygroscopic law expressed by the second conclusion; and with the unknown existence of that law, to suspect the march of his hygrometer: which accidental complication I shall explain hereafter. 65. When I published those results of my experiments and observations, M. de Saussure rejected them; not from having made new experiments that had confirmed his opinions; but because he conjectured inversely, that my theory resulted from a fallacious march of my hygrometer: and the well-earned reputation of that celebrated philosopher engaged me to undertake every experiment that could help me to detect on which side was the error. I have related, in the first part of this Paper, some of those experiments; and now, for their application, as well as for giving an account of some others, I shall follow more particularly M. de Saussure's process. 66. In a large glass vessel, containing (as I have mentioned above) a manometer and his hair-hygrometer, which vessel he had previously reduced to a known small distance from extreme dryness, M. de Saussure introduced from time to time a piece of wet cloth, which he weighed both before he put it into the vessel, and when he took it out. The successive increases in the quantity of vapour resulting from that process were indicated; on the manometer, by successive increases in the quantity of the inclosed elastic fluids, which caused the quicksilver to ascend more and more in that instrument; and on the hygrometer, by successive expansions of the hair. The maximum of evaporation was clearly indicated by the manometer; for, during every lasting temperature, the quicksilver, after having ascended to a certain point, remained fixed at that point, notwithstanding a longer stay of the wet cloth; and by repeating that operation at different temperatures, M. de Saussure determined the quantities of evaporated water that, in a given space, and by a given temperature, produced the maximum of evaporation. That single determination, from its great consequences not yet generally attended to, would be sufficient to fix the celebrity of its author, as I have already expressed many times in other works. 67. But with respect to a different conclusion from the same experiments, no less important to natural philosophy, I have also said, that M. de Saussure's hygrometer may have misled him. We have seen, that the manometer indicated, by an indubitable symptom, the maximum of evaporation; for here the immediate cause is clearly known, namely, the quantity of elastic fluids; and it is evident, that the quicksilver must ascend in the instrument in proportion to that quantity, and stop when it ceases to increase. But it is not the same as to the indications of the hygrometer in respect of moisture: M. de Saussure found himself that they were far from proportional to the intensities of their cause; and in the last stage of his experiments, though these indications did not vary much by the different maxima of evaporation, they however varied in the space of 1 or 2 degrees. But as those small differences on the point where the hygrometer stopped in different experiments, did not follow apparently any law conformable to the temperature, M. de Saussure considered them as small anomalies, unavoidable in hygroscopic substances, and of little consequence on a scale of 100 degrees; therefore, laying aside that circumstance, he could have no doubt, that, in every temperature, the maximum of evaporation in a close space was synonymous with the maximum of moisture in that space; while, from my experiments, these two supposed identical expressions may differ $\frac{1}{3}$, and sometimes $\frac{1}{4}$, of the real scale of moisture, which is the case in the temperature of only $75^\circ$ or $80^\circ$. 68. Let us now suppose for a moment, that the above hygroscopic law, and the march that I attribute to the hair hygrometer, are real. In that case, if, during a constant maximum of evaporation, the temperature varies from $32^\circ$ to $80^\circ$, moisture will diminish $\frac{1}{5}$ and even $\frac{1}{4}$ of the whole; or, in other words, the state of the medium will be distant by so much from that in which a new introduction of vapour would be followed by a precipitation. But at the same time, in the whole of that period of moisture, the hair hygrometer is supposed to move only 1 or 2 degrees backwards and forwards, with frequent irregularities. Therefore, in the hypothesis, such a great change of moisture would be hardly suspected from those small deviations of the hair hygrometer, in which at first nothing appears to be regular; and thereby it is evident, that, by confining himself to those experiments, M. de Saussure could not discover those two important laws of hygrology and hygrometry, of which I have here only supposed the existence. 69. Let us suppose again, that the hair hygrometer had not existed before a certain number of other experiments; and that M. de Saussure, in his attempt to produce an instrument of that kind, had fixed on any of the slips made of fibrous vegetable or animal substances cut across the fibres, of which many hygroscopes had been made before in a coarser manner; and that, in every other respect, he had proceeded as he has done with the hair. In that case, having placed his hygrometer and the manometer in the same vessel, with a quantity of water sufficient for producing the maximum of evaporation in every common temperature, and observed also the points where both instruments stopped in different lasting temperatures, he then would have found; that the hygrometer indicated less and less moisture, at the same time that the manometer, by ascending more more and more as the evaporation increased by more heat, indicated that increase in the quantity of vapour, distinct from the expansions of the fluids; and that each of those instruments remained also fixed in that state of opposite changes, by every sufficiently durable change of temperature. The first observation of that phenomenon would have surprized him, as it did me, and it would have induced him also to try some other of those slips; and by finding the same phenomenon with every one of them, he would have been convinced, that it was a real law of moisture. Lastly, if he had known Mr. James Watt's observations on the great dryness of the steam of boiling water, as long as it remains in a space as warm as the water that produces it (a condition always understood when the general laws of evaporation are the object of inquiry); admitting with me, as it follows from his own theory, that such steam is no other than the same kind of vapour thus far mentioned, only rendered capable of a greater density by more heat; he would then have seen the importance of that hygroscopic law from its great extent. I have hardly any doubt, that extreme dryness would reign in a close place similar to Papin's digester, if there were a sufficient space above the water and a red heat; though that space would be filled with vapour to the maximum. This relates only to the hygroscopic law here in view, on which, in the supposed case, I do not think we could have had any controversy. 70. Lastly, let us suppose, that in order to try the effect of moisture on the substance he had first chosen, or on some other of the same kind of substances, but taken lengthwise, M. de Saussure had happened to try first one of those which, used in that manner, have a great retrogradation, as goose-quill and deal; and that, after having previously observed it in the open open air, he had inclosed it in the moist vessel, at a time when the place where it stood before had the degree of moisture corresponding with the stationary state of that hyroscope, he would then have observed a phenomenon as little expected as the former: for after that instrument had been inclosed in the moist vessel, it would have moved, by that increase of moisture, in the same direction as it had done in the open dry air when there was on the contrary an increase of dryness. Surprised no doubt at that phenomenon, M. de Saussure would have submitted his new instrument to more experiments; he would also have tried other threads, in which he would have found the same sort of march, only at various degrees; and if, in the course of those trials, he had submitted the hair to the same experiments, the smallness of its motions backwards and forwards, and their irregularities, would not have prevented him from discovering in it the same sort of march as he had then been used to see in other threads; and thereby, he would have abandoned the whole tribe of threads as unfit for the hygrometer. 71. The whole of that supposed course of experiments with slips and threads, is that which I have followed from the time I had abandoned the construction of my first hygrometer; which I did especially with the view of being able to try many substances. Therefore my theory was formed in consequence of the two above conclusions, which appear to me immediate, and such as M. de Saussure could not have drawn differently, if he had followed the same necessary steps: and now I will prove, moreover, that if it had not been for accidental circumstances in his own process, the hair alone would have engaged him by degrees to undertake the same experiments. 72. M. de Saussure's first hygrometers, having their index at the top of the frame, could be plunged into water; and he tried tried that method for fixing their point of extreme moisture, as I had done for my first hygrometers. But in those trials he observed, that while his instruments stood in water, their indications remained undetermined within a space of four or five degrees; and attributing that irregularity to a friction of the hair with water, he thought it necessary to change my method, to that of placing the instrument in a moist medium, which he produced by means of a glass jar, wet on the inside, and inverted over water. In this method, the situation of the index became indifferent; and, for some particular reason, he placed it at the bottom of his new hygrometers, which then could not be plunged into water. This last circumstance was merely accidental; however, we shall see how much it has influenced his opinions in respect of extreme moisture. 73. In the first account of his experiments, M. de Saussure did not enter into those particulars; but they are in his answer to me: and when I there saw, that he gave as a reason for having abandoned the immersion in water, that a strong adhesion of the hair to that liquid impeded the freeness of its motions, I found it more natural to assign to the instrument itself the unsteadiness of its index which he attributed to that cause. Indeed, in those first hygrometers, one end of the axis passed through a hole for the purpose of carrying the index outwards, which was a cause of much friction; that axis besides was loaded with the weight of pretty large pincers, holding the hair, and that weight was counterpoised on the other side. Lastly, the connexion of the hair with the index was produced by a silver lamina, which, though very thin, opposed some resistance in bending round the axis. Those are defects that M. de Saussure corrected afterwards; but they existed in the instruments which he plunged into water, and a weight of only only 3 grains was not sufficient to keep their index steady, either in water or anywhere; and this he observed himself. 74. Notwithstanding that natural explanation of the unsteadiness of M. de Saussure's first hygrometer when in water, I thought it necessary to try, in the same circumstance, some well-constructed hair hygrometer; therefore I made two, similar to the last of M. de Saussure's in every respect, except that of having their index at the top; and for the connection of the hair with the axis, I used a kind of pincers, not above half a grain in weight, with a hair-like bit of hemp, which I know does not alter in any sensible manner the march of the instrument. Now, these hair hygrometers, with their weight of only 3 grains, being put into water, follow in it their own laws, arrive and remain fixed at their once settled point, as well as any of my other hygrometers. This M. de Saussure would have also found, if in the improvement of his instrument the axis had remained at the top. 75. Before I explain the influence which that accidental circumstance of the place of his index has had on his opinions with respect to extreme moisture, I must mention another of the same kind which has contributed to the same effect. It is evident, both from theory and from M. de Saussure's own experiments related above (§ 66.), that a sufficient quantity of water in any part of a close vessel is the only requisite for producing in it the maximum of evaporation; but, with a view of accelerating that effect for the common purpose of fixing the point of extreme moisture on his hygrometer, he prescribes wetting the inside of the vessel, besides inverting it over water. He did not (and indeed he could not) foresee the consequence of that alteration in his first process; but in fact it was such as to prevent him from discovering, even with time, time, in those operations, what he would have seen immediately if his last hygrometers could have been plunged into water. 76. After the improvement of his hygrometers, their index was no more subject to that unsteadiness observed in the first; consequently they remained fixed under the moist vessel; but they did not fix at the same point every time; and it happened accidentally, from causes of irregularity in the vessel itself, that in some of the cases, when the hair was the longest, a precipitation of water happened (by some partial cooling) on some part of that vessel; and he took that appearance for a sure sign that there was a superfluous quantity of water in the inclosed air. From that accidental connexion of circumstances he concluded, that his hygrometer indicated two different states of the medium in respect of extreme moisture; one, correspondent to about 98 on that instrument, which he considered as real extreme moisture, or that state of the medium in which no more vapour could be introduced into it without a precipitation; the other, when such a precipitation took place, which he made correspondent to 100, or to the greatest length of the hair. Prepossessed afterwards with that opinion, when I published my comparative experiments of his hygrometer and mine, in which the latter stood sometimes at 80 when the former was at 98, he concluded from that circumstance, that while his hygrometer moved only 2 degrees by the utmost effect of a supersaturation of the medium, mine had 20 of those insignificant degrees. In this centers the whole of our disagreement, and I am now going to trace its cause in the accidental circumstances above described. 77. When I plunge my hair hygrometers into water, where, as I have said, they come to a fixed point, that point does not indicate the greatest length of the hair; for, on the contrary, that that thread is then shorter than it is most times under the moist vessel. This M. de Saussure would have seen, if he had not been prevented, by the situation of the index in his improved hygrometer, from trying again the effect of water on the hair; and that phenomenon alone would surely have given a different course to his ideas; especially he would not have supposed, that the hair lengthens 2 degrees more, by a super-saturation of the medium, or by the immediate contact of concrete water. 78. If, also, when he settled the manner of determining the point of extreme moisture on his hygrometers, M. de Saussure had retained the simplicity of the process he had used for his fundamental experiments, in which a piece of wet cloth had been sufficient for producing the maximum of evaporation in his large vessel; and, in consequence, had contented himself with inverting his glass jar over water, without wetting it on the inside, he would have avoided a great cause of deception which I am going to explain. In my first experiments on the comparative marches of our hygrometers, in which I followed M. de Saussure's prescription for the moist vessel, I found some anomalies which puzzled me. M. de Saussure himself took notice of them in the account I gave of those experiments, and attributed them to my instrument. I did not agree with him in that respect; but it was long before I could discover the real cause of those anomalies. The first step towards that discovery, was the reflecting on the uselessness of wetting the vessel on the inside, for the only purpose of producing in it the maximum of evaporation. That consideration engaged me to undertake a new course of the same experiments, with a glass jar merely inverted over water; and by that means, the greatest part of the real anomalies being removed, I observed clearly in the march of the hair, the combined combined effects of its own property, and of the hygroscopic law respecting evaporation which was known to me from other phenomena. 79. As for the cause of those anomalies which had been removed by the change of the process, the following phenomenon led me to discover it. I observed frequently, at times when my hygrometer, placed under the jar, stood at a considerable distance from its point of extreme moisture, that a very small diminution of heat was sufficient to cause, on the lower part of the vessel, the formation of a tarnished rim, extending one or two inches above the surface of the water, with a thin vanishing edge. Having reflected on that phenomenon, from the mechanism I assign to the operation of fire in the very act of evaporation I concluded, that in such a stagnant air every evaporating surface had an atmosphere of extreme moisture, which extended as indicated by the tarnished rim; and that it was only beyond that limit that reigned the other law, of a decreasing moisture, correspondent to the increasing maxima of evaporation by an increasing heat. That new law of evaporation offered evidently an adequate cause for explaining the anomalies observed in the wet vessel; for glass retains concrete water very imperfectly, and it runs down very soon from many of the places which had retained it. Consequently, under such a partially wet vessel, and differently so at different times, the instruments must be variously affected by scattered atmospheres of extreme moisture. 80. However, before I could trust that explanation, I wanted to submit it to some direct experiment; and I succeeded by a means which at the same time realized what M. de Saussure thought he had obtained, namely, to produce extreme moisture in a close space, during any common temperature, without any precipitation of water from the medium. This I have produced by means of a wire cage, 4 inches in diameter, covered with cotton cloth, having at the top a reservoir, by which the cloth is kept thoroughly wet for a long time; which cage besides is inclosed in the glass jar inverted over water. In that apparatus, though in summer-time, every hygrometer, either thread or slip, moves and fixes itself, not so speedily, but else exactly as if it was plunged into water, without any supersaturation of the inclosed medium, or precipitation of water on the hygroscopic substance. 81. We may see now that the idea of two sorts of extreme moisture is without any foundation. In order to enforce the necessity of taking the point of extreme moisture in the air brought to that state, and not in water, M. de Saussure says, "That the hygrometer is not to measure the moisture of water, but that of the air." This at first appears plausible; however, in reality, moisture is no more to be considered in water itself, than heat in the fluid called fire. Water is the cause of moisture, as fire is the cause of heat; but those effects are not produced on their causes; it is on other substances. Therefore, if some hygroscopic substances are placed in a medium which has attained extreme moisture, and, in proportion as they take water from it, the loss of that water be constantly repaired by a new evaporation, they will receive by degrees in such a medium, without any precipitation, as much water as if they were plunged in water itself; for the limit is their capacity, which I have explained in § 19. This is the same theory that I had expressed in my first Paper on Hygrometry; and it is completely confirmed by the above experiment, with the discovery of this new hygroscopic law: "That in a stagnant air, every evaporating surface has an atmosphere of extreme moisture, which "which extends in a space of few inches, diminishes rapidly, "and does not interfere beyond that limit with the other laws "of moisture." 82. I shall now explain, by an example, what is the fundamental deviation of an hair hygrometer, introducing in it, for a moment, those two singular points 98 and 100, which, in M. de Saussure's experiments, were a very natural cause of mistake. A hair hygrometer and mine, being in a close vessel, at a time when the temperature, sensibly constant, shall be but little above 32; if moisture is first introduced into that vessel, so as to bring the hair hygrometer, by a very slow direct motion, to 98, my hygrometer will stop between 70 and 75; and both instruments will be fixed, if moisture and heat remain the same. Let moisture then be made to increase very slowly, till the hair hygrometer has attained its point 100; mine will have arrived at 80; and they again will remain at those points as long as, with the same temperature, the same quantity of vapour shall remain in the vessel. Lastly, let a sufficient or superfluous quantity of water be introduced into the vessel, the hair hygrometer will retrograde to 98, and mine proceed to 100, at which points they will stop, whatever be the quantity of water; and they will remain fixed, as long as the heat shall not increase. This explains the riddle of the singular point 98, or of a certain point, various in different hair hygrometers, various even, at different times, in the same individual, at which that instrument stands with very different degrees of moisture; consequently, its little motions round that point may create great deception, especially from the nature of organized substances, on which now I shall add a few words. 83. The above is the fundamental march of the hair hygrometer, such as it is on the whole, and as it would be con- stantly, if no other cause interfered; but it is subject to disturbing anomalies, which become worthy of attention within that small critical space which I have described. The texture of organized hygroscopic substances occasions a friction between their parts, when, by the changes of moisture and heat combined with their elasticity, they undergo changes in their respective positions; whereby they hardly can completely return to the very same arrangement, though with the same external circumstances; even at extreme moisture, when, there being less friction between their particles, the greatest part of the disturbances produced in their former motions are restored. To that general cause of irregularity is added a particular cause, when those substances are in the state of hygrometers; this is the influence of two opposite forces acting constantly on them; one, the tendency of their component parts to remain united; the other, a weight or spring which tends to separate them. Certain accidental arrangements of their component parts give them more power to resist the action which tends to separate them; and those arrangements are very changeable, by the alternate introduction and expulsion of moisture, by a long stay within a small compass of variations, and by more or less heat. This is a large field of facts and speculations, not uninteresting in itself, but on which I must not dwell: what I have said of those causes is sufficient to account for the anomalies to which, more or less, every hygrometer is subject. But whereas in the slips, those anomalies create only some irregularities in the observations, without any deceiving consequence in respect of the laws of moisture, they may deceive when they happen to interfere in the critical part of the march of some threads; for instance, if, by a certain accidental arrangement of the constituent parts in a slip of whalebone, there happens to be some tenths of a degree difference, from one experiment to another, on its point of extreme moisture, that anomaly cannot be of any consequence on the determination of what must be considered as that state in the medium; but if it happens to the hair, which, in approaching extreme moisture, has but very small motions, it may reverse those which it had had naturally (as I have observed it sometimes) and become a cause of deception. 84. I have now explained, how mere accidental circumstances have been the cause of a difference in the ideas that M. de Saussure and I had formed on what is to be understood by extreme moisture in every case; and I am going to illustrate the whole of that subject by a singular fact. An hygrometer made with a box thread, or a thin fascicle of the fibres of that wood, being placed in open air, next to a hair hygrometer, or to most of the other instruments of that sort, moves in a contrary way from them; but we may lay aside that circumstance, by supposing, that the numbers, marked on the dial of the first, are increasing in the opposite direction from those of the other instruments. Let us then suppose, that some experimental philosopher had chosen the box thread for his hygrometer; with him I should have fallen into no controversy on the point of extreme moisture; for, either under the moist vessel, or in any other case approaching extreme moisture, his hygrometer would have moved like mine. But the box thread, at approaching extreme dryness, first relents much its pace, then becomes stationary, and afterwards retrograde; by which property, with the concourse of some accidental circumstances as have happened in M. de Saussure's experiments, the very same questions that I have examined with so much labour in respect of extreme moisture, only because of the hair hygrometer, would have been transported to the point of extreme treme dryness; which, however, till now, has created no doubt. 85. That singular thread illustrates also another point, closely connected with the retrogradation of its tribe, but not to be confounded with it; I mean the recoil. The common cause of both phenomena is, two opposite effects produced by changes of moisture, on the length of threads. The retrogradation in the march, comparatively to that of moisture, is produced by one of the effects, which before was surpassed by the other, becoming predominant; and the recoil, or a returning back a part of a first stride when moisture changes suddenly, is produced, by one of the effects, that on the fibres themselves, being performed sooner than that on the sort of reticle formed by the fibres. Now, the box thread having its retrogradation at approaching extreme dryness, there also the recoil becomes sensible: it appears in the first modifications of that thread when placed in my dry vessel, by motions backwards and forwards, as it happens to some other threads, when taken out of water, or exposed to any other sudden change of moisture. The box thread having a slow motion, no recoil is clearly distinguishable in its common march; whereas the hair and the quill-thread, which, in appearance, are very quick, have generally a very disturbed motion when moisture changes suddenly. I have seen them, when in a free air, and happening to be in their stationary state, moving quickly one way in a space of 1 or 2 degrees, and then recoiling slower, sometimes to the same point where they were before, while my hygrometer underwent a steady change, which was in the first direction of theirs. This phenomenon, of a complete recoil in the stationary state of quick threads, is similar to the recoil of the index in those glass frames that I have described in § 59. of this Paper, which have a compensation for the changes of heat by a thin brass lamina: this being sooner affected than the glass rods, by sudden changes of heat, the index moves first one way; then it recoils completely, by the change being operated later in the glass rods. 86. The experiments briefly related in this Paper will, I hope, be sufficient to answer the following question, which has been made to me by some observers of the two principal hygrometers here compared: "Why does the hair hygrometer, when exposed to the open air in daytime, come so often near its point of extreme moisture, while the whalebone hygrometer almost never comes within 30 degrees distance of that point in summer, and very seldom within 20 in winter, even in rainy weather (if preserved from rain)?" The answer, according to the results of these experiments, is this: "The general march of the hair hygrometer is much decreasing, comparatively to equal increases of moisture; that march forwards, ends in a stationary state, and is followed by a small retrogradation; while the whalebone hygrometer has constantly a march, if not proportional, at least constantly similar to that of moisture itself." 87. There remains an object of inquiry, which is, a determination of those ratios here generally expressed. I have explained, in the first part of this Paper, the difficulties of that object, and what help may be found in comparing the marches of hygrometers with the acquisitions of weight of their substances, of which process I gave some examples; and here I shall relate similar experiments on hair, whalebone, box, and aloes-pitta. But as I have already described the whole process, and the manner of calculation of that class of experiments, I shall only give here the results of these last. 88. I must, however, first explain another reduction that I have added to the former. In the first table which I have given of those experiments, I followed the immediate division of my instruments, in which o corresponds to extreme dryness, and 100 to extreme moisture. But first, under that form, the point called 100 by M. de Saussure would not appear in its true light, as it means the greatest length of the hair; while the point 100 of my scale designs the state of that thread in water, where it has a little retrogradation. Under that form also most part of the terms in the observation on the box thread would be negative, since it moves very long in a contrary direction to the other hygrometers. For these reasons, instead of calling o the point of extreme dryness, and that of extreme moisture 100, I have, in the following tables, applied the first of those denominations to the smallest length of each substance, and the last to their greatest length. That reduction produces no difference in the proportions between the terms, and none in the terms themselves in respect to slips; as in these, the greatest length is always observed by extreme moisture, and the smallest by extreme dryness. II. Table II. Table of comparative changes in the weight and in the length of the same substances, by the same increases of moisture, correspondent to the march of the slip of whalebone from 5 to 5 of its degrees. | WHALEBONE | HAIR | ALOES-PITTA | |-----------|------|-------------| | Increases of the weight in shavings | March of the flip | Increases of the weight in a mass of hair | March of the hair | Increases of the weight in a mass of pitta | March of the thread of pitta | | Extr. dryness | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | | 6.0 | 5 | 4.8 | 15.7 | 6.0 | 20.6 | | 11.8 | 10 | 8.8 | 29.0 | 11.8 | 35.1 | | 17.3 | 15 | 12.5 | 40.0 | 17.3 | 51.6 | | 22.2 | 20 | 15.9 | 50.4 | 22.2 | 57.6 | | 26.8 | 25 | 19.1 | 59.7 | 26.8 | 75.6 | | 31.2 | 30 | 22.2 | 67.5 | 31.2 | 71.9 | | 35.2 | 35 | 26.6 | 74.4 | 35.2 | 76.3 | | 39.7 | 40 | 29.0 | 79.3 | 39.7 | 83.0 | | 44.0 | 45 | 32.0 | 83.3 | 44.0 | 86.6 | | 48.1 | 50 | 35.0 | 88.0 | 48.1 | 93.6 | | 52.1 | 55 | 38.2 | 90.0 | 52.1 | 96.5 | | 57.1 | 60 | 43.3 | 92.8 | 57.1 | 94.7 | | 61.7 | 65 | 49.8 | 94.1 | 61.7 | 98.2 | | 66.3 | 70 | 55.3 | 95.4 | 66.3 | 100.0 | | 71.9 | 75 | 61.9 | 97.0 | 71.9 | 99.2 | | 77.6 | 80 | 68.7 | 100.0 | 77.6 | 98.2 | | *83.2 | *85 | *76.0 | *99.5 | *83.2 | *96.8 | | *88.8 | *90 | *84.0 | *99.2 | *88.8 | *94.1 | | *94.4 | *95 | *92.0 | *98.6 | *94.4 | *91.9 | | In water | *100.0 | *100.0 | *97.7 | *100.0 | *88.3 | 89. In the above, and in the following experiments, the operation of successively introducing moisture into the vessel was stopped, when the slip of whalebone was at 80, as beyond that term the smallest difference in the temperature between the parts of the apparatus creates great anomalies; therefore the fol- Vol. LXXXI. H h h lowing following terms in the three columns of weights, which are marked with an *, have only been added (as I have explained for the former Table) with the view of having a common modulum between the changes of weight and the marches of the other instruments. But the observed terms remain in their original proportions, and from these we may see, that the march of the slip of whalebone does not differ much from the successive increase of weight in its own substance; and that when taken out of the apparatus, and immediately plunged into water, it proceeds in the same direction as before, till it has attained its fixed point, while the hair takes, comparatively with the increase of weight of its own substance, great strides in the beginning of its march, and very small steps in the latter part of it, before 100; and then retrogrades a little, when taken out of the apparatus, and plunged into water. We see besides in that table, that the thread of aloes-pitta, which at first takes still greater strides than the hair, has, after a longer stationary or undetermined state, a determined beginning of retrogradation at the same time that its own substance continues to acquire weight in the apparatus, and continues that retrograde march when, being taken out of the vessel, it is plunged into water. 90. The following table will farther illustrate these characteristic differences of slips and threads, the ascertaining of which was so essential to hygrometry. III. Table ### III. Table of experiments on the comparative changes in the weight and the length of the same substance by increase of moisture. | Slip of whalebone | March of the slip | Increases of the weight in shavings | March of the thread | |-------------------|------------------|------------------------------------|--------------------| | Extr. dryness | | | | | 0 | 0.0 | 0.0 | 72.8 | | 5 | 4.5 | 7.3 | 87.2 | | 10 | 9.5 | 12.8 | 93.2 | | 15 | 14.5 | 17.8 | 97.8 | | 20 | 20.0 | 22.6 | 100.0 | | 25 | 25.7 | 27.3 | 95.9 | | 30 | 31.5 | 31.8 | 92.7 | | 35 | 38.0 | 38.5 | 88.6 | | 40 | 45.5 | 44.5 | 79.9 | | 45 | 51.5 | 49.7 | 70.3 | | 50 | 56.5 | 54.8 | 63.9 | | 55 | 61.2 | 59.1 | 57.3 | | 60 | 65.7 | 63.1 | 51.0 | | 65 | 69.7 | 66.4 | 47.5 | | 70 | 73.7 | 69.6 | 40.9 | | 75 | 77.7 | 76.6 | 31.4 | | 80 | 81.5 | 80.0 | 21.7 | | 85 | 85.9 | * 85.0 | 16.0 | | 90 | 90.5 | * 90.0 | 10.4 | | 95 | 95.5 | * 95.0 | 5.1 | | In Water | 100 | * 100.0 | 0.0 | We see in this table the slip of box following, in its increases of length, the increase of weight in the shavings of the same wood, nearly in the same manner as the slips of whalebone, quill, and deal, follow those of their own shavings; while the thread of box, after having gained some length by decreasing... decreasing steps, begins soon to shorten, at the same time that its substance continues to imbibe water; being thus the shortest, when it cannot receive any more water in its pores. That excess of the hygroscopic phenomenon of threads cannot but throw a full light on the nature of those hygrosopes. 91. I am now going to assemble, in two tables, the comparative marches of all the threads, and of all the slips, which I have hitherto submitted to that regular course of experiments, laying aside many more of each class, the marches of which I only know from common observations. The next table shall contain the experiments on threads; in the number of which are two thin natural bodies, which in that respect are similar to the hair; one, an animal substance, is a very thin porcupine quill; the other, a vegetable, is a thin stem of gramen. ### Table of the correspondent marches, by the same increase of moisture, of different threads or vegetable and animal substances taken lengthwise. | Porcupine quilt | Whale bone | Hair | Gut | Aloes | Gooch | Deal | Granae | Bor. | Slip of whalebone | |-----------------|-----------|------|-----|-------|-------|------|--------|-----|------------------| | Ext. dryness | | | | | | | | | | | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 72.8 | | 18.0 | 12.0 | 15.6 | 9.7 | 20.6 | 37.0 | 33.2 | 26.8 | 87.4| 5 | | 34.0 | 29.9 | 29.4 | 19.2| 35.1 | 66.6 | 54.8 | 48.4 | 93.2| 10 | | 48.8 | 39.9 | 40.9 | 26.8| 51.6 | 78.7 | 74.9 | 67.1 | 97.8| 15 | | 62.3 | 50.8 | 50.5 | 37.0| 57.6 | 88.0 | 84.6 | 76.6 | *100.0| 20 | | 73.3 | 58.8 | 59.2 | 47.1| 75.6 | +93.4 | 89.8 | 83.9 | 95.9 | 25 | | 81.0 | 65.3 | 68.8 | 57.3| 71.9 | 97.2 | 93.8 | 90.5 | 92.7 | 30 | | 86.8 | 70.8 | 73.0 | 67.4| 76.3 | 99.0 | 96.9 | 95.1 | 88.6 | 35 | | 90.8 | 76.1 | 78.3 | 75.6| 83.0 | 94.4 | 94.3 | 98.6 | 79.9 | 40 | | 93.0 | 81.4 | 82.1 | 82.9 | 86.6 | 96.2 | 97.7 | *100.0 | +70.3 | 45 | | 95.0 | 85.4 | 86.1 | 87.8 | 93.6 | 99.0 | *100.0 | 98.8 | 63.9 | 50 | | 94.5 | 88.4 | 88.8 | +91.6 | 96.5 | 95.3 | 94.6 | 98.0 | 57.3 | 55 | | 97.0 | 90.8 | 91.6 | 94.7 | 94.7 | 97.2 | 97.0 | 97.2 | 51.0 | 60 | | 96.5 | 92.8 | 93.8 | 96.3 | 98.2 | 98.2 | 94.6 | 96.2 | 45.7 | 65 | | 96.5 | 95.1 | 95.6 | 97.8 | *100.0 | *100.0 | 93.0 | 94.8 | 40.9 | 70 | | 95.0 | 97.1 | 97.2 | 98.7 | 99.2 | 99.0 | 91.4 | 92.6 | 31.4 | 75 | | 97.0 | 98.1 | +98.0 | *100.0 | 98.2 | 98.2 | 89.0 | 89.8 | 21.7 | 80 | | 98.0 | 99.1 | 100.0 | 98.7 | 96.8 | 97.2 | 86.9 | 86.5 | 16.0 | 85 | | 98.6 | +99.6 | *100.0 | 96.8 | 94.1 | 95.8 | 84.6 | 84.0 | 10.4 | 90 | | 99.1 | *100.0 | 99.3 | 94.5 | 91.5 | 94.4 | 81.9 | 80.9 | 5.1 | 95 | | In water | 100.0 | 99.5 | 98.3 | 91.8 | 92.5 | 79.0 | 77.0 | 0.0 | 100 | --- 92. There 92. There the porcupine quill shews no retrogradation; however, consistent with its tribe, it had some in other experiments. Its last steps have the unsteadiness of the stationary state, and thereby are subject to anomalies. From the same cause, none of the other threads have exactly the same steps in any two experiments, though on the whole their march remains essentially the same. The march here given of the hair hygrometer comparatively with mine, is the mean result of three experiments, with three different sets of instruments; one of the hair hygrometers that I have employed was sent to me by Mr. Paul, of Geneva, and its point of extreme moisture was determined in a fog. The small and changeable retrogradation of the thread of whalebone and of hair might have been overlooked, were it not for other threads in which the retrogradation begins before that period where the state of moisture is difficult to ascertain; but from these threads, that phenomenon is placed in a clear light, which is reflected on the others. I have marked with an * the greatest elongation of each of them, and with a + a point near which their elongation begins, and to which they return at last. These signs will guide the eye in the above table, which shews clearly, that no thread can be trusted to for the hygrometer. Table ### Hygrometry **Table of the correspondent matches of stirs, or of fibrous vegetable and animal substances taken across the fibres, and of such as have no flexible fibres.** | Goole | Porcupine | Ship of | Box | Deal | Ivory | Ivory | Tortoise | Horn | Horn | |-------|-----------|---------|-----|------|-------|-------|----------|-----|-----| | quill. | quill. | whalebone. | breadthwise. | lengthwise. | breadthwise. | lengthwise. | breadthwise. | lengthwise. | | Ext. dryness | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | | 4.8 | 4.8 | 5 | 4.5 | 5.4 | 6.1 | 8.3 | 11.0 | 9.5 | | 9.7 | 8.8 | 10 | 9.5 | 11.2 | 12.7 | 16.6 | 21.5 | 18.5 | | 14.4 | 12.0 | 15 | 14.5 | 16.5 | 18.7 | 24.6 | 31.5 | 27.5 | | 19.2 | 17.0 | 20 | 20.0 | 21.9 | 24.9 | 31.5 | 38.5 | 37.0 | | 23.9 | 23.4 | 25 | 25.7 | 27.2 | 30.4 | 37.6 | 45.4 | 46.5 | | 28.5 | 29.4 | 30 | 31.5 | 32.7 | 35.4 | 43.6 | 51.9 | 54.5 | | 33.3 | 36.0 | 35 | 38.0 | 38.3 | 41.9 | 49.7 | 58.3 | 62.1 | | 38.3 | 41.4 | 40 | 45.5 | 43.7 | 47.4 | 56.3 | 63.8 | 68.7 | | 42.9 | 45.4 | 45 | 51.4 | 49.2 | 53.5 | 62.4 | 69.0 | 72.3 | | 47.4 | 49.8 | 50 | 56.5 | 54.6 | 58.5 | 67.4 | 72.8 | 77.0 | | 52.4 | 54.8 | 55 | 61.2 | 59.9 | 63.5 | 71.6 | 76.4 | 79.7 | | 56.9 | 59.7 | 60 | 65.7 | 64.9 | 68.0 | 76.1 | 79.4 | 84.3 | | 61.9 | 64.4 | 65 | 69.7 | 69.7 | 72.1 | 79.1 | 82.4 | 86.4 | | 67.2 | 68.5 | 70 | 73.7 | 74.5 | 76.1 | 82.9 | 84.9 | 88.4 | | 72.2 | 73.5 | 75 | 77.7 | 79.0 | 80.1 | 86.7 | 88.2 | 90.2 | | 77.8 | 78.9 | 80 | 81.5 | 83.5 | 84.5 | 90.4 | 91.2 | 92.0 | | 82.8 | 83.9 | 85 | 85.9 | 87.5 | 87.8 | 92.4 | 93.8 | 94.0 | | 88.2 | 88.9 | 90 | 90.5 | 92.0 | 92.0 | 94.5 | 96.2 | 96.1 | | 94.0 | 94.4 | 95 | 95.3 | 96.0 | 96.0 | 97.5 | 98.6 | 98.1 | | In water | 100.0 | 100.0 | 100.0 | 100.0 | 100.0 | 100.0 | 100.0 | 100.0 | --- 93. This 93. This last table is the most important, as it contains a class of hygrometers which possess in common the following first requisites for an hygrometer; 1st, that they may indicate, without any illusion, both extreme dryness and extreme moisture; 2ndly, that they move constantly in the same direction as moisture itself; 3rdly, that they move always when moisture changes. It should seem as if the march of the slip of horn taken lengthwise, from its very decreasing progression, came very near that of the thin porcupine quill; but, as I have said, among the steps of the latter there are accidental retrogradations, and it sometimes has a final one; and I have never observed that disposition in the former, which, in its last small steps, follows constantly the motions of every other slip. 94. The agreement of all the slips in this last respect is a very essential circumstance in hygrometry, as it assures us, that we cannot mistake the cases when moisture is extreme in the atmosphere; a very important point for discovering the nature of many meteorological phenomena. No slip will create deception in that respect; while, on the contrary, every thread may deceive in dubious cases, and even create great error, if, unknown to the observer, it happened to be in the beginning of its elongation. There was, however, a question to be decided in that respect, namely, whether or not a great moisture in the medium was a cause of alteration in the march of any hygroscope, by producing in its substance a sudden irregular lengthening. That accidental question is answered in the negative by all the hygrometers of both classes: for, in respect of the threads, instead of lengthening suddenly in that period of moisture, they have then a retrograde motion, either continuing or only beginning; and as for the slips, they, by lengthening in the same period, only follow their former laws: the slips which which, comparatively to that of whalebone, have at first small steps, and which consequently move in an increasing progression, continue only to follow that progression; and those which at first have greater steps, and consequently a decreasing march, have then small steps conformable to their individual law; therefore, none of those hygoscopes of both classes have any sudden start, produced by any degree of moisture in the medium, or by the application of concrete water; each of them follows, from one end to the other of its scale, its own progression; and in respect of slips, moisture is never extreme in the ambient medium, as long as, in their respective progressions, they have not attained their greatest length. 95. Our common hygrometer must then be made of one of the slips; but with that great dissimilarity observed in their marches, which of them shall we choose as indicating the real march of moisture? None as yet from that consideration, which I do not even think a primary one. It is true, that if we trust to the increases of weight in those substances, as being a means of ascertaining the real progress of moisture in the ambient medium, the mean rate of six experiments of that kind related in this Paper, give the preference to the slip of whalebone; but this I do not yet consider as decisive, farther than in what relates to the comparative marches of slips and threads; however, as my reasons of doubt on a more absolute conclusion cannot be expressed in a transitory manner, I must lay them aside for the present. 96. But, as I have said before, this is not what ought to determine our choice on the substance of a common hygrometer, since the observations themselves are distinct from the consequences to be drawn from them. Let us suppose the case (which I do not give up) that, with time and researches, some process be found by which known quantities of moisture may be successively produced in the medium itself. The use of that process for hygrometry will be, as M. de Saussure has begun to do it, to observe, on some hygrometer, the successive effects of those known quantities of moisture, from which may be formed a table of the correspondence between the equal degrees of the scale of the chosen instrument, and the real quantities of moisture in the medium; and that table will serve to correct as well past as future observations made with that instrument. Therefore it matters not what that hygrometer shall be, provided it is convenient in other respects. Let us then examine which of the slips possesses the most essential properties of an hygrometer, such as should be in common use for comparative observations, and to which consequently future discoveries in respect of the real proportions between the quantities of moisture itself would be applied. 97. Steadiness is surely a first requisite for such an instrument; and in that respect no slip comes in competition with that of whalebone. That property was the first motive of my choice; and as an instance of it I shall only mention, that I have just now plunged into water an instrument of that sort, of above ten years standing, which is come to its point of extreme moisture as if it had been fixed yesterday; for, without regard to the distance of observations, there may be between them a difference of some tenths of a degree. Some other slips may be brought to a certain degree of steadiness by studying what is the degree of stretch which they may bear; but that attention is not necessary for the slip of whalebone: if, for instance, when its point of extreme moisture has been fixed while it was stretched to a certain degree, that stretch is much much increased, it will acquire some absolute length; but it will be steady again for a new point taken then in water. 98. Another property of the slip of whalebone, which at first should seem contradictory to the former, is its great expansibility, in which also it surpasses all the substances which I have tried. Such a slip lengthens above one-eighth of itself from extreme dryness to extreme moisture, which produces many advantages in the construction and observation of that instrument. In respect to observation, when it is exposed to the wind, the difference between the chords of the arches of its bends and its real length is so small, comparatively with its hygroscopic variations, that the indetermination of its index will remain confined in a space of one or two degrees, when it becomes impossible to observe hygrometers whose substance has but little expansion. Lastly, of all the substances which I have reduced to slips, none is so easily made thin and narrow as whalebone. I have found means for producing easily such slips of it as, with a length of eight inches, weigh only about \(\frac{1}{6}\)th of a grain, and are thereby as quick as is convenient in other respects. All those distinctive properties of the slip of whalebone seem to point out an hygroscopic substance fit for our common hygrometer. Description of the whalebone hygrometer. 99. I have now only to describe the construction of that instrument as I have fixed it after a long experience. The fig. 1. (Tab. IX.) shews its form for common use. Some of those instruments are of the same size as the figure, and they may easily be made smaller, but commonly they are half as large again in every dimension. Their frame will sufficiently be known from the figure, therefore therefore I shall confine myself to the description of some particulars. The flip of whalebone is represented by \(a, b\); and at its end \(a\) is seen a sort of pincers, made only of a flattened bent wire, tapering in the part that holds the flip, and pressed by a sliding ring. The end \(b\) is fixed to a moveable bar \(c\), which is moved by a screw for adjusting at first the index. The end \(a\) of the flip is hooked to a thin brass wire; to the other end of which is also hooked a very thin silver gilt lamina, that has at that end pincers similar to those of the flip, and which is fixed by the other end to the axis by a pin in a proper hole. The spring \(d\), by which the flip is stretched, is made of silver gilt wire; it acts on the flip as a weight of about 12 grains, and with this advantage over a weight (besides the avoiding some other inconveniencies of this) that, in proportion as the flip is weakened, in its lengthening by the penetration of moisture, the spring, by unbending at the same time, loses a part of its power. The axis has very small pivots, the shoulders of which are prevented from coming against the frame, by their ends being confined, though freely, between the flat bearing of the heads of two screws, the front one of which is seen near \(f\). The section of that axis, of the size that belongs to a flip of about 8 inches, is represented in fig. 2.; the flip acts on the diameter \(a, a\), and the spring on the smaller diameter \(b, b\). 100. Another construction of the same instrument is represented, of half its dimensions, by fig. 3. (Tab. X.) The essential parts of that hygrometer are the same as in the other; therefore I shall only mention the differences adapted to the use of fixing it out of a window. The figure represents the manner in which it is fixed, with its dial turned half-way towards the observer, by a proper head of the hooks which hold it at the top and the bottom. bottom. The steady part of the instrument is represented by fig. 4.; it is partly an half tube, cut longitudinally through the axis, and having sidewise two vertical rows of large holes. A whole tube goes over that half one; for which purpose the piece \(a\), \(a\), is taken off by unscrewing the female screw \(b\) which belongs to the part \(d\); this last is a small open tube, cut outside in screw. The large external tube has also two vertical rows of holes, at such a distance from one another, that, when one of them corresponds to one of the rows of the half tube, the other is in front. The two opposite positions that tube can be brought to are in order that the instrument may be placed either side of a window, and each of those positions is determined by one end of a cut at the bottom of the tube in \(a\), fig. 3. which then holds against the steady pin \(c\), fig. 4. The rows of holes of the tube are to be turned towards the room, to prevent the rain from falling on the slip; and the dial being inclosed in a box with a glass in front, no rain can get into the instrument. It must also be fixed in a place not much exposed to the sun, or be screened from it without preventing the circulation of the air. The communication of the slip with the external air through the rows of holes and the open bottom \(d\), fig. 4. is sufficient for that class of observations. By the manner of its being hooked, it may be easily taken off for carrying elsewhere; and, if a quick observation is wanted, the tube also may easily be taken off. I have the honour of presenting one of those instruments to the Royal Society; and, as it is very desirable that some hygrometer be added to the other meteorological instruments usually observed, I wish this may deserve a place in their Observatory for that purpose.