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Therm, on
the snow.

Therm. in the air.

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thermometer, the air at that elevation was found to be pretty constantly 4 warmer than at the station below.

Exper. 7. The result of this trial appeared more remarkable than any thing which had bitherto occurred. M. W. lowered the pole till the thermometer was brought down within half a foot of the ballustrade, but keeping it still a few inches to windward of the buildings; and by this means it was found that the air here was never colder than + 10°. On the ballustrade there happened to be 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 boar.frost, there was always a remarkable fall of the mercury, not less than 6°. In shifting the instrument from the pole to the ballustrade, it was commonly laid on some hoar-frost i 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.froast were also made to touch the upper part of the ball ; which was done by pushing them on with a long frozen straw. Exper. 8. When the thermome.

At Night. ter, taken from the pole as in last At 9

.+ 5 + 6 experiment, was laid on pieces of 94..+5..+ 8 stone, from which the hoar.frost 10 +6 + 8 had been brushed away for some 101..+6..+ 10 time before, the mercury sunk but 11 .+6..+ 9 very little by such a change of situ. 11į .+ 5..+ 8 ation. Next night, being that of 12 + 5 + 8 Sunday January 23, the thermo. 124 ..+4..+ 7 meters were placed in their former 1 Morn.+4..+ 8 station below, when they pointed as 11..+4..+ 8 annexed.

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 Mr.W. made another experiment with a view to the evaporation, not so liable to objec. tions as those of the bellows and the fan, as follows.

Exper. 9. When the thermometer in air at the lower station kad coutracted a considerable film of frozen matter all over the

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ball, it was swung round at the end of a pack.thread, about a yard and a half long. On stopping the motion at the expiration of two minutes, and making the servant who waited approach quickly with a lighted candle, he found the mercury had got up 2o. In this experiment, which was repeated four times with the same re. sult, care was always taken to keep the instrument to windward of their bodies, and of the lighted candle.

The two following experiments afford some grounds for believ. ing that no kind of evaporation was going on at the time the re. markable excess of cold in the snow and hard frost was observed.

Exper. 10. On Sunday morning, January 23, before one o'clock, Mr. W. repeated the experiment with the metal speculum which was tried here in 1768. A large spare metal of a 2.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, to. wards 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 21 hours. 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 multiplied, till at length it was mostly covered by 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 2d 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 am. bient 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 ex. periment, 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 hoar.frost.

Erper. Jl. 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 hoar-frost seemingly as soon as each body had time to cool 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 ex. cess of cold, at present treated of, depending on 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 any thing 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 on principles different from evaporation or chemical solution. At the same time some of the experiments show that a free communication between the hoar-froast and external air, perhaps while in motion, is ne. cessary; but in what manner this promotes the refrigeration does not as yet appear.

It would be going too far were we to conclude, from the experi. ments related above, “ that very cold air is never disposed to deposit its contents except on 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 slates and other thinner parts about a house whitened with hoar-frost, when the walls and more solid parts of the build. ing 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 paked for a long time; and, in gene. ral, any thin or detached body, capable of being easily cooled, at. taches hoar.frost the soonest.

In favour of this general position, the following remarkable case lately occurred. 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 sto:e 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, to give the rail more stability. On Sunday morning, February 13, when there was a slight frost accompanied with a fog, it was entertaining to observe how the hoar-frost had settled during the night on these bars. Very little was to be seen on the flat sides, but a great deal on 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 the parapet, it was observed, that the fringe of hoar-frost on 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 pil. lar. And it was also observed, that the few thicker and stronger bars was less frioged at the corners and were quite free much far. ther 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: while they, on the other hand, resisted this change by draw. ing heat from every neighbouring source which offered it, namely, from the parapet, from the pillars in the middle, and from the pil. lars at both ends immediate adjoining 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. Where. ever 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 no where else.

Several other instances were found quite of the same kind with that of the rail. Among the rest, a figure of a unicorn in the 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 hour.frost. Besides this kind of hoar.frost which joined itself to bodies by a regular arrangement, there was some of a different sort found on the uppermost surface of such bodies as were fully exposed to the open air. But this al. ways 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.

(Phil. Trans. Abr. Vol. xiv.

CHAP. XXXVIII.

EVAPORATION.

We found reason to conclude, in the preceding chapter, that the water of the atmosphere exists in the state of vapour. We are indebted to the experiments of Saussure and Deluc for much of our knowledge of the qualities of vapour. It is an elastic invisible fluid like common air, but lighter ; being to common air of the same elasticity, aecording to Saussure, as 10 to 14, or, according to Kirwan, as 10 to 12. It cannot pass beyond a cer. tain maximum of density, without a corresponding increase of temperature, otherwise the particles of water which compose it unite together, and form small visible particles, called vesicular vapour; which is of the same specific gravity with atmospherical air. It is of this vapour that clouds and fogs are composed. This maximum increases with the temperature; and at the heat of boiling water is so great, that steam can resist the whole pressure of the air, and exist in the atmosphere in any quantity.

We have seen formerly, that when water is heated to 212°, it boils, and is rapidly converted into steam; and that the same change takes place in much lower temperatures; but in that case the evaporation is slower, and the elasticity of the steam is smaller. As a very considerable proportion of the earth's surface is covered with water, and as this water is constantly evaporating and mixing with the atmosphere, in the state of vapour, a precise determina. tion of the rate of evaporation must be of great importance in

VOL, IV.

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