An Account of a Most Extraordinary Degree of Cold at Glasgow in January last; Together with Some New Experiments and Observations on the Comparative Temperature of Hoar-Frost and the Air Near to It, Made at the Macfarlane Observatory Belonging to the College. In a Letter from Patrick Wilson, M. A. to the Rev. Nevil Maskelyne, D. D. F. R. S. and Astronomer Royal

XXVI. An Account of a most extraordinary Degree of Cold at Glasgow in January last; together with some new Experiments and Observations on the comparative Temperature of Hoar-frost and the Air near to it, made at the Macfarlane Observatory belonging to the College. In a Letter from Patrick Wilson, M. A. to the Rev. Nevil Maskelyne, D. D. F. R. S. and Astronomer Royal. Read April 20, 1780. SIR, THE observations and experiments related in the sequel were made here during the great frost in the course of last month. I shall be extremely happy if they contribute any thing to your entertainment, being induced to send you the account in consequence of your having on a former occasion so politely invited me to a further correspondence. If the paper shall appear to you deserving the notice of the Royal Society, you will please to consider it as entirely at your own disposal. Most part of the time my father was confined to his room with bad health, and could not therefore gratify his curiosity by taking a part in the observations. I have now the pleasure of acquainting you, that he is quite well again, and on this occasion he begs to be remembered to you with best compliments. On Tuesday, Jan. 11th, there was a flight frost, and on the evening of that day we had a fall of snow to the depth of twelve inches. Next day the cold continued to increase, but so gradually, that at sun-set Fahrenheit's thermometer pointed only to $22^\circ$. Upon returning home to the College from a visit about midnight, I hung out a very accurate thermometer at a high North window, where it soon after pointed to $6^\circ$. At this time the air was very still and serene, and the barometer stood at 30 inches. Thursday Thursday morning, Jan. 13, 1 o'clock, thermometer pointed to \[ \begin{align*} 1\frac{3}{4} & \quad +6 \\ 2\frac{1}{2} & \quad +4 \\ 3 & \quad +6 \\ 4 & \quad +3 \\ 4\frac{1}{2} & \quad +2 \\ 5 & \quad +2 \\ 5\frac{1}{2} & \quad 0 \end{align*} \] At 6 o'clock this morning I carried the thermometer over to the Observatory Park, and when it was there laid down upon the snow the mercury sunk to gr. 13 below 0. Your friend Professor Anderson soon after arriving, we repeated the observation with an excellent thermometer made by Mr. Ramsden, and it agreed exactly with the other. At this time we thought it unnecessary to stay abroad so long in the cold as to try the temperature of the air by hanging up the thermometers, especially as we imagined that this had been done more readily, and as truly, by taking the degree from the surface of the snow which had been exposed exposed to the open air during the night; but upon reflecting afterwards on the snow at the Observatory being so much below 0, the greatest cold of the air at the College, and having on other occasions found a difference of only 4° at most in air at these two stations, I was led into a suspicion that the snow might perhaps have been so far cooled down by an evaporation at the surface. With a view to this opinion I projected the experiment with the bellows described below, by which I was not without expectations of producing a still more remarkable fall of the thermometer when lying on the snow. Before dinner this day I met with Dr. Irvine, to whom I communicated the above observations, and the intention of trying the evaporation; for that snow and ice did actually evaporate in some circumstances seemed sufficiently proved by an experiment made here by my father in 1768, which is related in the Philosophical Transactions, vol. Lxi. p. 326. Dr. Irvine approved much of the proposal, and most obligingly agreed to spend the night at the Observatory, that we might, by a regular course of observations, ascertain the difference of temperature, and try whether evaporation was really the cause of it. All the afternoon the cold was very intense, and at seven o'clock at night the thermometer at the high North window pointed to gr. 0. At eight Dr. Irvine and I repaired to the Observatory, and made choice of a station at a sufficient distance from the house, and to the windward, as a light air was felt coming from the East. Here we laid down two thermometers on the snow with their balls half immersed, and hung up other two freely exposed to the air at two feet and a half from the surface. In the following observations the interruption of the series from 2½ to 6½ o'clock was owing to an accident having befallen one of the thermometers whilst the other was employed in the trials, of which an account is subjoined. Thursday, Thursday evening, Jan. 13. Below 0. | Time | Therm. on snow pointed to (gr.) | Therm. in air to (gr.) | |------|---------------------------------|------------------------| | 9 | -14 | -2 | | 10 | -14 | -4 | | 11 | -17 | -6 | | 11½ | -18 | -6 | | ½ o'clock Friday morning | -20 | -8 | | 1 | -23 | -7 | | 1½ | -22 | -8 | | 2 | -22 | -9 | | 2½ | -21 | -8 | | 3 | | -9 | | 3½ | | -10 | | 4 | | -12 | | 4½ | | -12 | | 5 | | -12 | | 5½ | | -12 | | 6 | | -14 | | 6½ | -22 | -13 | | 7 | -22 | -13 | | 7½ | -22 | -13 | | 8 | -19 | -10 | EX- EXPERIMENT I. At half past one o'clock, when the thermometer pointed to $-22^\circ$, the snow contiguous to the ball was blown upon for two minutes by a pair of hand-bellows, held with the pipe nearly horizontal, and half a foot above the surface of the snow. The bellows had been lying out on the snow to cool from the time we first came over; and, in order to promote their cooling, they were now and then wrought in the open air. Care was also taken to stand to leeward of the thermometer, and to extend the bellows as far as possible from the body in the time of blowing. We were surprised to find, however, notwithstanding all our precautions, that the thermometer at the end of the experiment had got up no less than $10^\circ$, for it now pointed only to $-12^\circ$. In this experiment the nozzle of the bellows was held about six inches from the thermometer, but the blast, though moderate, frequently drifted away the snow from the ball. EXPERIMENT II. At half past two o'clock, a bread-basket was filled with snow, taken up near the ground at $+14^\circ$. The contents being relatively so warm, the basket was placed to leeward of the common station, and the thermometer laid on the surface of this snow. At 3 o'clock in the morning, thermometer on the basket pointed to \[ \begin{align*} 3\frac{1}{2} & \quad -15 \\ 4 & \quad -16\frac{1}{2} \\ 4\frac{1}{2} & \quad -18 \\ 5 & \quad -18 \\ 5\frac{1}{2} & \quad -18 \\ 6 & \quad -18 \end{align*} \] **EXPERIMENT III.** At four o'clock in the morning, when the thermometer in the basket had got down to \(-16^\circ\), a piece of thin fir plank about a foot square was laid on the snow, upon which was placed a small plate of tin which accidentally lay at hand. Upon this was laid one of the thermometers which had been hanging in the air. At 5 o'clock, therm. on the plate pointed to \[ \begin{align*} 5\frac{1}{2} & \quad -16 \\ 6 & \quad -18 \end{align*} \] At seven this morning Professor Anderson made us a visit, and had the satisfaction of seeing the thermometers whilst they indicated such an extraordinary degree of cold. cold. During the whole time not a cloud was perceivable, but there was a faint haze in the air when viewed towards the horizon. There was little or no tremor in the atmosphere, which made the stars to shine with a full and steady light like that of the planets. Many of the citizens, who had thermometers hung out at their windows in different places of the town, found them pointing several degrees below 0 at nine o'clock in the morning. On the afternoon of this day, being Friday, Jan. 14th, the air became much warmer, and the barometer had now fallen four-tenths. Next day a thaw came on, and continued for some time. As the above experiment with the bellows favoured so little the opinion, that the difference of temperature was caused by evaporation, I began to think, that it might be owing to a continued descent of cold air somewhere in the neighbourhood, which cooled the snow by sweeping along its surface in so thin a sheet as not to affect the air a little higher up. But as this solution of the phenomenon was so arbitrary, and unsupported by any facts, I wished for another opportunity of making further experiments, and of enquiring into circumstances still more attentively. A good occasion offered on Saturday, Jan. 22. The frost, which before this time had again returned, became on this night very keen; and a good deal of the former snow yet remaining on the ground, the following observations and experiments were made whilst I was favoured with the company and assistance of Dr. ADAIR CRAWFORD, who passed the whole of this night at the Observatory. On Sunday morning, Jan 23, \[ \begin{array}{c|c|c} \text{Time} & \text{Therm. on snow pointed to} & \text{Therm. in air} \\ \hline \frac{1}{2} \text{o'clock} & +4 & +14 \\ \frac{3}{4} & +5 & +14 \\ 1\frac{1}{4} & +4 & +11 \\ 1\frac{3}{4} & +3 & +11 \\ 2\frac{1}{4} & +3 & +11 \\ 2\frac{3}{4} & +3 & +11 \\ 3\frac{1}{2} & +1 & +8 \\ 4 & +1 & +6 \\ 4\frac{1}{2} & 0 & +6 \\ 5 & -1 & +5 \\ 5\frac{1}{2} & -1 & +6 \\ 6\frac{1}{4} & -1 & +6 \\ 7 & 0 & +6 \\ 7\frac{1}{2} & -3 & +5 \\ 7\frac{3}{4} & -2 & +5 \\ 8\frac{1}{2} & +1 & +7 \\ \end{array} \] EXPERIMENT IV. This night, instead of blowing on the snow, we fanned it by means of a sheet of brown paper fitted to the end of a long slender stick. This apparatus was previously cooled by lying on the snow, and in fanning we took care to stand to leeward of the thermometer. The effect was, that the mercury rose nearly to the same degree given by the thermometer in air at the same time. EXPERIMENT V. At three quarters past one o'clock, when the thermometer on the snow pointed to +3°, it was screened by two sheets of brown paper set up on their edges, and so inclined against one another as to stand. The paper had been previously cooled by lying on the snow. At a quarter past two o'clock the thermometer thus sheltered pointed to +9°. This experiment was afterwards repeated with the same event. EXPERIMENT VI. We next went up to the leads of the East wing of the Observatory. Here we hung a thermometer to the hook of a long pole, and raised it in the air about twenty-four feet from the ground, and at the same time inclined the pole over the balustrade, so as to put the instrument fully to windward of the house. Upon suddenly lowering the pole, after half an hour, and examining the thermometer, the air at that elevation was found to be pretty constantly $4^\circ$ warmer than at the station below. EXPERIMENT VII. The result of this trial appeared more remarkable than any thing which had hitherto occurred. We lowered the pole till the thermometer was brought down within half a foot of the balustrade, but keeping it still a few inches to windward of the building, and by this means it was found that the air here was never colder than $+10^\circ$. Upon the balustrade there happened to be several several detached bodies which had attracted a very thick hoar-frost. When the thermometer was taken off the hook of the pole, and laid on this hoar-frost, there was always a remarkable fall of the mercury, not less than $6^\circ$. Both Dr. Crawford and I were much struck with this fact, and attended to it very carefully. In translating the instrument from the pole to the ballustrade, it was commonly laid on some hoar-frost three quarters of an inch deep, which had settled on a piece of thin board which had been for years exposed to the weather. Some fragments of the hoar-frost were also made to touch the upper part of the ball; which was done by pushing them on with a long frozen straw. **EXPERIMENT VIII.** When the thermometer taken from the pole as in last experiment was laid on pieces of stone, from which the hoar-frost had been brushed away for some time before, the mercury sunk but very little by such a change of situation. Next night, being that of Sunday Jan. 23, the thermometers were placed in their former station below, when at 9 o'clock 9 o'clock at night, therm. on snow pointed on Therm. in air to | Time | Thermometer | Thermometer | |------|-------------|-------------| | 9 | +5 | +8 | | 10 | +6 | +8 | | 10½ | +6 | +10 | | 11 | +6 | +9 | | 11½ | +5 | +8 | | 12 | +5 | +8 | | 12½ | +4 | +7 | | 1 | Monday morning | +4 | | 1½ | | +8 | From these observations it appears, that the cold now was very moderate when compared to that of the 14th, and somewhat more moderate than that of the preceding night. Experiment 7th was again repeated with a similar result, though the difference of temperature was not now so great. This night I made another experiment with a view to the evaporation, not so liable to objections as those of the bellows and the fan. This shall now be described. **EXPERIMENT IX.** When the thermometer in air at the lower station had contracted a considerable film of frozen matter all over over the ball, it was swung round at the end of a pack-thread, about a yard and a half long. Upon stopping the motion at the expiration of two minutes, and making the servant who waited approach quickly with a lighted candle, I found the mercury had got up $2^\circ$. In this experiment, which was repeated four times with the same result, care was always taken to keep the instrument to windward of our bodies, and of the lighted candle. This night the thermometer, when put under the snow close to the ground, pointed to $+18^\circ$. Both now and on the preceding night there were no clouds, though the air, as on the 14th, was a little thick towards the horizon, and there was always a very perceptible and constant motion of it from East to West. On this and the preceding night the Moon, a little past the full, shone very bright, and the mercury was stationary at 30 inches. On Sunday, Feb. 13. we had a slight frost accompanied with a thick fog. This occasioned a vast settling of hoar-frost upon the small branches and twigs of trees, and upon all thin and detached bodies exposed to the open air. The weather continued much in this state on Monday, when, at ten o'clock forenoon, I repeated the seventh experiment on the ballustrade, but now found no difference whatever in the temperature of the hoar-frost and the air in its neighbourhood. The thermome- ter in both cases pointed to +22°. About 11 o'clock the sun-shine broke through the fog, when the temperature of the hoar-frost was as quickly affected as that of the air. The air on this day was quite still, and equally cold both at the upper and under stations. It may also be here observed, that no snow lay on the ground. The ninth experiment was also repeated, but the thermometer was not now in the least affected by swinging it round. Neither on this occasion, nor when the experiment was first made, did any of the frozen matter appear to have parted from the ball. If we consider the excess of cold in the snow which Dr. Irvine and I first observed as a phenomenon of the same kind with that described in the seventh experiment, and proceeding from the same cause, it is manifest that neither the one nor the other can be accounted for by any previous cold state of the air, according to an hypothesis alluded to in the beginning of this letter: for in the seventh experiment the air at the ballustrade was never colder than +10°, and yet the hoar-frost there was at the same time found in several instances as cold as +2°. That both these phenomena are of the same kind appears extremely probable from this consideration, namely, that when the snow upon the fields was attentively examined, the surface was found quite covered over ever with the same sort of hoar-frost which was attached to other bodies which had been long exposed. The two following experiments afford some grounds for believing that no kind of evaporation was going on at the time the remarkable excess of cold in the snow and hoar-frost was observed. **EXPERIMENT X.** On Sunday morning, Jan. 23, before one o'clock, Dr. Crawford and I repeated the experiment with the metal speculum which was tried here in 1768. A large spare metal of a two-foot telescope was laid out to cool, after which a film of ice was imparted to its polished surface by breathing on it four or five times. It was then exposed as before, and in half an hour the whole film disappeared in the way of evaporation. But when the experiment was again repeated, and a thicker film imparted, some of this, towards the middle of the speculum, remained fixed, and would not go off after long exposure. The speculum was next warmed, and its polished surface made quite clean, and then laid out for two hours and a half. Before the expiration of this time it began to draw frozen matter from the air, which settled all over the polished surface in long parallel lines which gradually gradually multiplied, till at length it was mostly covered with a thin film resembling a spider's web. The evaporation shewn in the first part of this experiment was probably owing to the speculum not having been sufficiently cooled when the film was first communicated to it from the lungs, and to its being further heated by that very operation. In the second part of the experiment the evaporation seems to have stopped when the heat in the metal which favoured the process was exhausted; that is, when the speculum had arrived at the temperature of the ambient air, for after that no heat could pass from the metal in order to contribute to the evaporation. But from the last part of the experiment, the true disposition of the air at that time, relative to bodies as cold or colder than itself, seems to be determined, namely, that of giving out or depositing hoarfrost. EXPERIMENT XI. On Sunday night, January 23, several things were laid out at the Observatory, such as sheets of brown paper, pieces of boards, plates of metal, glasses of several kinds, &c. which all began to contract hoarfrost seemingly as soon as each body had time to cool down down to the temperature of the air. The sheets of brown paper being so thin acquired it soonest, and when beheld in candle-light they became beautifully spangled over by innumerable reflections from the small crystals of hoar-frost which had parted from the air. Evident symptoms of the same tendency of the air to deposit occurred on all the former nights of observing, by which the tubes of the thermometers were so much stained, that it required some attention to keep that part which corresponded to the scale quite clear. These experiments indeed rather favour the opinion of the excess of cold at present treated of depending upon a principle the very reverse of evaporation. But till opportunities offer in this or in a colder climate of making more experiments, it will be too early to say anything decisive concerning the nature or extent of a cooling process which has so recently come under observation. All that can at present be affirmed is, that in certain circumstances such a process goes on, and that it depends probably upon principles different from evaporation or chymical solution. At the same time some of the experiments shew that a free communication betwixt the hoar-frost and external air, perhaps whilst in motion, is necessary; but in what manner this promotes the refrigeration doth not as yet appear. Q q q 2 Thoſe Those gentlemen whose opportunities have led them to know how far the philosophy of heat hath, of late times, been extended by the great discoveries of Dr. Black, Dr. Irvine, and Dr. Crawford, will be most interested in the views which offer from the foregoing experiments. If at first sight there appears any thing adverse to certain general principles already established, yet we may rest satisfied that this is but in appearance only; as the perfect method of induction, pursued by these philosophers, leaves no room to apprehend that any future discovery will militate against their conclusions. In the further prosecution of this subject, and in whatever way it may be cleared up, it is probable, that we shall meet with a fine instance of the congruity of nature in all her operations, and of the stability of those general laws, which have been derived from a cautious observance of the rules of the experimental philosophy. I am, &c. The following observations which relate to the disposition of the air in giving out hoar-frost may be here subjoined. It would be going too far were we to conclude from the experiments related above, "that very cold air is never never disposed to deposit its contents except upon bodies as cold or colder than itself." And yet that this is frequently the case seems probable from a number of common appearances. We often find, after a night of frost, the flutes and other thinner parts about a house whitened with hoar-frost, when the walls and more solid parts of the building remain quite free. In like manner the smaller branches and twigs of trees often acquire this frozen ornament, when the main branches and trunk remain naked for a long time; and, in general, any thin or detached body, capable of being easily cooled, attach hoar-frost the soonest. In favour of this general position, the following remarkable case lately occurred, which at the time I shewed to Dr. Reid, in consequence of whose approbation I am induced at present to bring it into view. Between the public library and the buildings of the new court there is a long rail composed of bars of cast iron, but divided into two parts by two massy stone pillars which support the iron gate-way that leads into the garden. The bars are about six feet high and an inch square, and fastened with lead into a stone parapet below in the usual way. A few bars much larger are set in among the rest at regular distances, in order to give the rail more stability. On Sunday morning, Feb. 13, when we had the flight frost accompanied with a fog, it was entertaining to observe how the hoar-frost had settled during the night upon these bars. Very little was to be seen upon the flat sides, but a great deal upon the angles, by which means from the top downward every bar was garnished with four fringes, which made the whole rail look very gay and ornamental. Running the eye along the foot of the bars near to the parapet it was observed, that the fringe of hoar-frost upon the corners stopped short about twelve inches from the bottom, and that so much of every bar was entirely free. Two bars next the house and two next the library were likewise perfectly clear of it from top to bottom. One bar next the pillar of the gate was quite free, and the second had contracted but little. The same thing precisely may be said of the two bars contiguous to the other pillar. And it was also observed, that the few thicker and stronger bars were less fringed at the corners, and were quite free much farther above the parapet than the others. It is manifest, that during the night the air surrounding the bars must have been constantly endeavouring to make them as cold as itself, whilst they, on the other hand, resisted this change by drawing heat from every neighbouring source which offered it, namely, from the parapet, from the pillars in the middle, and from the pillars at at both ends immediately adjoining to the library, and to the house in the new court: for these bodies from their great bulk must have been but very little cooled in the course of the night. Wherever the air seems to have got the better in this struggle, as at the angles of the bars, which evidently must be the parts the soonest cooled, there we find that the hoar-frost was deposited, but nowhere else. Several other instances were found quite of the same kind with that of the rail. Among the rest, a figure of an unicorn in stone, which stands within the college, had resisted the attacks of the air all to the tip of his horn, which accordingly was the only part distinguished by a patch of hoar-frost. Besides this kind of hoar-frost which joined itself to bodies by a regular arrangement, there was some of a different sort found upon the uppermost surface of such bodies as were fully exposed to the open air. But this always lay scattered like very thin flakes of meal, or hair-powder, and was found to proceed from minute parts, mostly columnar, previously formed in the air, falling down by their own gravity.