An Account of a New Kind of Baroscope, Which May be Called Statical; And of Some Advantages and Conveniencies It Hath above the Mercurial: Communicated, Some While Since, by the Honourable Robert Boyle

PHILOSOPHICAL TRANSACTIONS. Munday, July 2. 1666. The Contents. An Account of a New kind of Baroscope, which may be call'd Statical; and of some Advantages and Conveniences it hath above the Mercurial; communicated by Mr. Boyle. The Particular Observations of the Planet Mars, formerly intimated to have been made by Mr. Hook in February and March last. Some Observations made in Italy, confirming the former; and withall fixing the Period of the said Planet's Revolution. Observations lately made at London, of the Planet Jupiter: as also of Saturn. A Relation of a sad Effect of Thunder and Lightning. An Account of some Books, lately publish'd; videl. The Relations of divers Curious Voyages, by Mons. Thevenot: A Discourse about the Cause of the Inundation of the Nile, by Mons. de la Chambre; both French: De Principiis & Ratiocinatione Geometricarum, Contra Fastum Professorum Geometriae, by Mr. Hobbes: King Salomons Pourtraiture of Old Age, by J. Smith, M.D. An Account Of a New kind of Baroscope, which may be called Statical; and of some Advantages and Conveniences it hath above the Mercurial: Communicated, some while since, by the Honourable Robert Boyle. So for the New kind of Baroscopes, which, not long agoe,* I intimated to you, that my haste would not permit me to give you an account off; since your Letters acquaint me, that you still design a Communicating to the Curious Curious as much Information, as may be, in reference to Baroscopes; I shall venture to send you some Account of what I did but name (in my former Letter) to you. Though by a Passage, you may meet with in the 19th and 20th Pages of my Thermometrical Experiments and Thoughts, you may find, that I did some years agoe think upon this New kind of Baroscope; yet the Changes of the Atmosphere's Weight not happening to be then such, as I wish'd, and being unwilling to deprive myself of all other use of the exactest Ballance*, that I (or perhaps any man) ever had, I confess to you, that successive avocations put this attempt for two or three years out of my thoughts; till afterwards returning to a place, where I chanc'd to find two or three pairs of Scales, I had left there, the sight of them brought it into my mind; and though I were then unable to procure exacter, yet my desire to make the Experiment some amends for so long a neglect, put me upon considering, that if I provided a Glass-buble, more than ordinary large and light, even such Ballances, as those, might in some measure perform, what I had tried with the strangely nice ones above-mention'd. I caused then to be blown at the Flame of a Lamp some Glass-bubles as large, thin and light, as I could then procure, and choosing among them one, that seem'd the least unfit for my turn, I counterpoised it in a pair of Scales, that would loose their Equilibrium with about the 30th part of a Grain, and were suspended at a Frame. I placed both the Ballance and the Frame by a good Baroscope, from whence I might learn the present weight of the Atmosphere. Then leaving these Instruments together; though the Scales, being no nicer than I have express'd, were not able to shew me all the Variations of the Air's weight, that appear'd in the Mercurial Baroscope, yet they did what I expected, by shewing me variations no greater, than alter'd the height of Quicksilver half a quarter of an Inch, and perhaps much smaller, than those: Nor did I doubt, that, if I had had either tender Scales, or the means of supplying the Experiment with convenient accommodations, I should have discerner scerned far smaller Alterations of the Weight of the Air, since I had the pleasure to see the Buble sometimes in an equilibrium with the counterpoise; sometimes, when the Atmosphere was high, preponderate so manifestly, that the Scales being gently stirr'd, the Cock would play altogether on that side, at which the Buble was hung; and at other times (when the Air was heavier) that, which was at the first but the Counterpoise, would preponderate, and, upon the motion of the Ballance, make the Cock vibrate altogether on its side. And this would continue sometimes many daies together, if the Air so long retain'd the same measure of gravity; and then (upon other changes) the Buble would regain an equilibrium, or a preponderance; so that I had oftentimes the satisfaction, by looking first upon the Statical Baroscope (as for distinctions sake it may be call'd) to foretell, whether in the Mercurial Baroscope the Liquor were high or low. Which Observations though they hold as well in Winter, and several times in Summer (for I was often absent during that season) as the Spring, yet the frequency of their Vicissitudes (which perhaps was but accidental) made them more pleasant in the latter of these seasons. So that, the matter of Fact having been made out by variety of repeated Observations, and by sometimes comparing severall of those new Baroscopes together, I shall add some of those Notes about this Instrument, which readily occur to my memory, reserving the rest till another opportunity. And First, if the ground, on which I went in framing this Baroscope, be demanded, the answer in short may be; 1. That, though the Glass-buble, and the Glass-counterpoise, at the time of their first being weigh'd, be in the Air, wherein they both are weigh'd, exactly of the same weight; yet they are nothing near of the same bulk; the Buble, by reason of its capacious cavity (which contains nothing but Air, or something that weighs less than Air) being perhaps a hundred or two hundred times (for I have not conveniency to measure them) bigger than the Metalline counterpoise. 2. That according to a Hydrostatical Law (which you know I have lately had occasion to make out) If two Bodies of equal gravity, but unequal bulk come to be weigh'd in another Medium, they will be no longer equi- equiponderant; but if the new Medium be heavier, the greater Body, as being lighter in Specie, will loose more of its weight, than the lesser, and more compact; but if the new Medium be lighter than the first, then the bigger Body will outweigh the lesser: And this disparity, arising from the change of Medium's, will be so much the greater, by how much the greater inequality of bulk there is between the Bodies formerly equiponderant. 3. That, laying these two together, I consider'd, that 'twould be all one, as to the effect to be produced, whether the Bodies were weighed in Medium's of differing gravity, or in the same Medium, in case its (specific) gravity were considerably alter'd: And consequently, that since it appear'd by the Baroscope, that the weight of the Air was sometimes heavier, and sometimes lighter, the alterations of it, in point of gravity, from the weight, it was off at first counterpoising of the Buble of it, would unequally affect so large and hollow a Body, as the Buble, and so small and dense a one, as a Metallin weight: And when the Air by an increase of gravity should become a heavier Medium, than before, it would buoy up the Glass more than the Counterpoises; and if it grew lighter, than it was at first, would suffer the former to preponderate: (The Illustrations and Proof can scarce be added in few words; but, if it be desired, I may, God permitting, send you them at my next leisure:) And though our English Air be about a thousand times lighter, than water, the difference in weight of so little Air, as i; but equal in bulk to a Buble, seem'd to give small hopes, that it would be sensible upon a Ballance; yet, by making the Buble very large and light, I suppos'd and found the Event, I have already related. Secondly, The hermetically seal'd Glass-buble, I employed, was of the bigness of a somewhat large Orange, and weigh'd about 1. drachme and 10. grains. But I thought it very possible, if I had been better furnish't with conveniencies (wherein I afterwards found, I was not mistaken) to make (among many, that might be expected to miscarry) some, that might be preferable to this, either for capacity or lightness, or both; especially if care be taken, that they be not seal'd up, whilst they are too hot. For, though one would think, that it were advantagious tagious to rarify and drive out the Air as much as is possible, because in such seal'd Bubles the Air itself (as I have elsewhere shewn) has a weight; yet this advantage countervails not the inconvenience of being obliged to increase the weight of the Glass, which when it includes highly rarified Air, if it be not somewhat strong, will be broken by the pressure of the External Air, as I have sufficiently tried. Thirdly, I would have tried whether the Dryness and Moisture of the Air would in any measure have alter'd the weight of the Bubble, as well as the Variation of Gravity produced in the Atmosphere by other causes; but the extraordinarily constant absence of Fogs, kept me from making Observations of this kind; save that one morning early, being told of a mist, I sent to see (being myself in bed) whether it made the Air so heavy as to buoy up the Bubble; but did not learn, that that mist had any sensible operation on it. Fourthly, By reason of the difficulties and casualties, that may happen about the procuring and preserving such large and light Bubles, as I have been lately mentioning; it may in some cases prove a convenience to be inform'd, That I have sometimes, instead of one sufficiently large Bubble, made use of two, that were smaller. And, though a single Bubble of competent bigness be much preferable, by reason that a far less quantity and weight of Glass is requisite to comprise an equal capacity, when the Glass is blown into a single Bubble, than when it is divided into two; yet I found, that the employing of two instead of one, did not so ill answer my expectations, but that they may for a need serve the turn instead of the other; than which they are more easier to be procured. And if the Ballance be strong enough to bear so much Glass, without being injur'd: by employing two or a greater number of large Bubles, the effect may be more conspicuous, than if only a single Bubble (though a very good one) were employed. This instrument may be much improved by divers Accommodations. As First, There may be fitted to the Ansa (or Checks of the Ballance) an Arch (of a Circle), divided into 15. or 20. deg. (more or less, according to the goodness of the Ballance) that the Cock resting over against these Divisions, may readily and without Calculation shew the quantity of the Angle, by which, when the scales propend either way, the Cock declines from the Perpendicular, and the beam from its Horizontall parallelism. Secondly, Those, that will be so curious, may, instead of the Ordinary Counterpoise (of Brass) employ one of Gold, or at least of Lead, whereof the latter being of equal weight with Brass, is much less in Bulk, and the former amounts not to half its bigness. Thirdly, These parts of the Ballance, that may be made of Copper or Brass, without any prejudice to the exactness, will, by being made of one of those Metals, be less subject, than Steel, (which yet, if well hardened and polish'd, may last good a great while) to rust with long standing. Fourthly, Instead of the scales, the Buble may be hung at one end of the Beam, and only a Counterpoise to it at the other, that the Beam may not be burthen'd with unnecessary weight. Fifthly, The whole instrument, if placed in a small Frame, like a square Lanthorn with Glass-windows, and a hole at the top for the Commerce of the internal and external Air, will be more free from dust, and irregular agitations; to the latter of which, it will otherwise be sometimes incident. Sixthly, This instrument being accommodated with a light Wheel and an Index (such as have been applyed by the excellent Dr. Chr. Wren to open Weather glasses, and by the ingenious Mr. Hook to Baroscopes) may be made to shew much more minute variations, than otherwise. Seventhly, And the length of the Beam, and exquisitiveness of the Ballance, may easily, without any of the foregoing helps (and much more with them) make the instrument far exacter, than any of those, I was reduced to employ. And to these Accommodations divers others may be suggested by a farther consideration of the nature of the thing, and a longer practice. Though in some respects this Statical Baroscope be inferior to the Mercurial; yet in others it has its own advantages and conveniencies above it. And 1. It confirms ad oculum our former Doctrine, that the falling and rising of the Mercury depends upon the varying weight of the Atmosphere; since in this Baroscope it cannot be pretended, that a Fuga vacui, or a Funiculus, is the cause of the changes, we observe. 2. It shews, that not only the Air has weight, but a more considerable one, than some Learned men, who will allow me to have prov'd, it has some weight, will admit; since even the variation of weight in so small a quantity of Air, as is but equal in bulk to an Orange, is manifestly discoverable upon such Ballances, as are none of the nicest. 3. This Statical Baroscope will oftentimes be more parable, than the other: For many will finde it more easie, to procure a good pair of Gold-scales, and a Buble or two, than a long Cane feal'd, a quantity of Quick-silver, and all the other requisits of the Mercurial Baroscope; especially if we comprise the trouble and skill, that is requisite to free the deserted part of the Tube from Air. 4. And whereas the difficulty of removing the Mercurial Instrument has kept men from so much as attempting to do it, even to neighbouring places; the Essential parts of the Scale-Baroscope (for the Frame is none of them) may very easily in a little room be carried, whither one will, without the hazard of being spoil'd or injur'd. 5. There is not in Statical Baroscopes, as in the other, a danger of uncertainty, as to the goodness of the Instruments, by reason, that in these the Air is, in some more, and in some less perfectly excluded; whereas in those, that consideration has no place. (And by the way, I have sometimes, upon this account, been able to discover by our new Baroscope, that an esteem'd Mercurial one, to which I compared it, was not well freed from Air.) 6. It being, as I formerly intimated, very possible to discover Hydrostatically, both the bigness of the Buble, and the Contents of the cavity, and the weight and dimensions of the Glassie substance (which together with the included Air make up the Buble,) much may be discover'd by this Instrument, as to the Weight of the Air, absolute or relative. For, when the Quick-silver in the Mercurial Baroscope is either very high, or very low, or at a middle station between its greatest and least height, bringing the Scale-Barometer to an Exact Equilibrium; (with very minute divisions of a Graine,) you may, by watchfully observing, when the Mercury is risen or fallen just an inch, or a fourth, or half an inch &c. and putting in the like minute divisions of a Grain to the lighter Scale, till you have again brought the Ballance to an exquisite exquisit æquilibrium; you may, I say, determine, what known weight in the statical baroscope answers such determinate altitudes of the ascending and descending quicksilver in the mercurial. And if the balance be accommodated with a divided arch, or a wheel and index, these observations will assist you for the future to determine readily, by seeing the inclination of the cock or the degree mark'd by the index, what pollency the bubble hath, by the change of the atmospheres weight, acquired or lost. Some observations of this nature I watchfully made, sometimes putting in a 64th, sometimes a 32th, sometimes a 16th, and sometimes heavier parts of a grain, to the lighter scale. But one, that knew not, for what uses those little papers were, coming to a window, where my baroscopes stood, so unluckily shook them out of the scales, and confounded them, that he rob'd me of the opportunity of making the nice observations I intended, though I had the satisfaction of seeing, that they were to be made. 7. By this statical instrument we may be assisted to compare the mercurial baroscopes of several places (though never so distant) and to make some estimates of the gravities of the air therein. As if, for instance, I have found by observation, that the bubble, I employ, (and one may have divers bubbles of several sizes, that the one may repair any mischance, that may happen to another) weigh'd just a drachme, when the mercurial cylinder was at the height of 29½ inches (which in some places I have found a moderate altitude;) and that the addition of the 16th part of a gr. is requisite to keep the bubble in an æquilibrium, when the mercury is risen an 8th, or any determinate part of an inch above the former station: When I come to another place, where there is a mercurial barometer, as well freed from air as mine (for that must be supposed) if taking out my scale instrument, it appeare to weigh precisely a drachme, and the mercury in the baroscope there stand at just 29½ inches, we may conclude the gravity of the atmosphere not to be sensibly unequal in both those two places, though very distant. And though there be no baroscope there, yet if there be an additional weight, as for instance, the 16th part of a grain requisite to be added to the bubble, to bring the scales to an æquilibrium, it will appear that the air at this second place is, at that time so much heavier, than the Air of the former place was, when the Mercury stood at 29½ inches. But in making such comparisons, we must not forget to con- sider the Situation of the several places, if we mean to make Estimates not only of the weight of the Atmosphere, but of the weight and density of the Air. For, though the Scales will shew (as has been said) whether there be a difference of weight in the Atmosphere at the two places; yet, if one of them be in a Vale or bottom, and the other on the top or some elevated part of a Hill, it is not to be expected, that the Atmosphere, in this latter place, should gravitate as much, as the Atmosphere in the former, on which a longer Pillar of Air does lean or weigh. And the mention, I have made of the differing Situation of Places, puts me in mind of something, that may prove a another use of our Statical Baroscope, and which I had thoughts of making tryal off, but was Accidentally hindred from the op- portunity of doing it. Namely, that by exactly poising the Buble at the foot of a high Steeple or Hill, and carrying it in its close Frame to the top, one may, by the weight requisite to be added to Counterpoise there to bring the Beam to its Hor- izontal position, observe the difference of the weight of the Air at the bottom, and at the top; and, in case the Hill be high enough, at some intermediate Stations. But how far this may assist men, to estimate the Absolute or Comparative height of Mountains, and other elevated Places; and what other Uses the Instrument may be put to, when it is duly improved; and the Cautions, that may be requisite in the several cases, that shall be proposed, I must leave to more leisure, and farther Consideration. The Particulars. Of those Observations of the Planet Mars, formerly intimated to have been made at London in the Months of February and March A. 1665. To perform, what was promised Num. ii. of these Papers, pag. 198; 'tis thought fit now to publish the Particular Obser- vations, concerning the spots in Mars, and their motion, as they were made with a 36 foot Telescope, and produced in