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correspond with complete saturation at the minimum temperature; more especially from the summer to the winter solstice, a period of the year when the accumulated moisture is returning back, by the decline of temperature, from the vaporous to the liquid state. In the present paper, however, it is not so muth my design to illustrate this point, as to endeavour to shew that the hygrometic condition of the atmosphere exerts a considerable reaction upon the temperature by which it is induced; the humidity of the air producing, in this respect, upon its temperature, an effect analogous to that of the fly-wheel of the machine, which regulates and controls the operation of the moving power by which it is itself kept in motion.

Every person who is at all acquainted with the general doctrines of caloric, is aware of the fact, that liquids, during their transition to the state of vapour, always absorb a large proportion of the matter or substance of heat. This heat is not discoverable, in its combined state, by any of the ordinary means which we employ to detect the presence and intensity of that subtile agent. In the case of the vapour of water, it exists in union with the aqueous particles in what is called a latent or concealed state; but the characteristic energy of its nature is only suspended or neutralized, not destroyed. The vapour, with which it is united, cannot recover the liquid form, until the watery particles have been disengaged from the igneous fluid, which, being thus separated, resumes its wonted activity, and produces upon contiguous bodies, with which it enters into new combinations, the usual indications of its presence and operation. Thus, when a quantity of common air, completely saturated with humidity, at any particular temperature, is made to suffer sudden condensation, the extrication of heat from the compressed vapour is not only perceptible by the senses, but sufficiently great to set fire to inflammable substances, placed within the sphere of its activity. The effect may be ascribed in part, no doubt, to the compression of the air, and the change of capacity for caloric, which it undergoes by condensation; but it seems to be chiefly owing to the transition of the vapour to the state of water, in consequence of the reduction of volume it sustains, the effect being much more limited when dry air is subjected to the experiment. The conversion of vapour to the liquid condition being thus necessarily attended with the evolution of heat, even when the experiment is performed upon a small scale, where, a large portion of the effect is expended upon the sides of the vessel containing the vaporised air, it is

obvious, that the great aërial magazine which encompasses our globe, cannot yield up the moisture with which it is, at all times, more or less copiously charged, without having its temperature extensively affected by the change.* Hence, if, by any great physical cause, the temperature of the air should be exposed to a sudden reduction, the extent of the effect would not only be modified, but in a great measure counteracted by the extrication of heat which would thus take place. In conformity with these views, it may be inferred, that the greatest cold during the night should always be observed when the atmosphere is in its driest state; and, conversely, that when the air is extremely moist, there should be little difference between the temperature of the day and the night. This conclusion, which will go far to explain the cause of the various deflections in the isothermal lines of Humboldt, I have verified by a vast multiplicity of observations, both near the level of the sea, and in situations 2000 feet above it; during the extreme heat of summer, as well as the most intense cold of winter, when the air was loaded with vapour, not less than when it was almost entirely bereft of humidity; insomuch, that, by ascertaining the hygrometric state of the atmosphere in the evening, I have no difficulty in determining the minimum temperature of the ensuing night, the deviation seldom being beyond the limits within which the nicest thermometrical observations are made.

The agreement, however, which I have endeavoured to trace between the minimum temperature and the point of deposition, is, for obvious reasons, most remarkable from the end of July to the end of December, the temperature of the year being then on the decline, and rendering the relative humidity greater than

*The heat extricated by the condensation of vapour is beautifully exhibited by the following simple experiment:-Let a little moistened linen or paper be placed under a receiver, upon the plate of an air-pump, in which there is at the same time suspended a delicate thermometer; on exhausting the air, the receiver will soon be filled with invisible vapour, which will be more or less dense, according to the temperature of the apartment at the time of the experiment. In the course of two minutes, if the air-pump be a good one, the density of the vapour will reach its maximum state, for the temperature; that is, the space occupied by the vapour will be incapable of holding more moisture in solution,—consequently, when the stop-cock is turned, the air, which is allowed to enter the receiver from the outside, carrying along with it the portion of water which it retains at the time in the vaporous state, the whole of that vapour must suffer condensation. The sides of the receiver are accordingly suddenly bedimmed with moisture, and the thermometer at the same instant rises 5 or 6 degrees. This experiment affords a fine illustration of the well known fact, that the formation of a cloud in the zenith of a clear and serene sky, never fails to raise the temperature of the air at the surface of the earth 2 or 3 degrees.

during the other half of the year; but at no season is the deviation so great as not to indicate a mutual connexion between them. It may also be proper to remark, that, after the temperature of the night reaches the point of deposition, which it seldom fails to do, the check to its farther diminution operates most effectually at the higher temperatures; because the quantity of vapour which passes into the liquid state being greater, it exerts a corresponding influence over the thermal state of the air, and thus prevents the lowest temperature of the night from ever being greatly depressed below the point of deposition. In the month of July, for example, when the mean point of deposition may be taken in this latitude at 45°, the quantity of moisture in the air is 0.2099 grains in 100 cubic inches; whereas, in the month of December, when the mean point of deposition is 15° lower, the quantity of moisture in the same volume of air is only 0.1278 grains. So that a depression of 1° of temperature in the former case, would cause nearly double the quantity of vapour to pass to the liquid state, and evolve a proportional quantity of caloric, to counteract any tendency (whether from radiation or any other cause,) which the air might have to a farther reduction of temperature. I shall not occupy the pages of this Journal with a tedious comparison of numerical results in support of the general positions which I have laid down, but confine myself to a few interesting details of some observations which I made, under circumstances peculiarly well fitted to subject the theory to a severe trial. These I shall briefly describe :

Having occasion, during the month of August 1822, to fix my residence for some time at Amulree*, a situation in the Highlands of Perthshire remarkable for the dryness and salubrity of its air, I carried along with me a number of good meteorological instruments, for the purpose of making such observations on the hygrometric state of the air, in that elevated part of the country, as might tend either to confirm or overturn my hypothesis. The state of the weather, which proved extremely variable, was highly favourable to the object I had in view, the thermometer having ranged, in that short interval, from 63° to 26°, and exhibited greater fluctuations than I have ever observed in the month of August. On the thirteenth the day was hazy, and the coincidence, the following night, between

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Amulree is, by my barometrical observations, 935 feet above the level of the sea.

the minimum temperature and the point of deposition, as indicated by my formula, was exact. The following day the humidity of the air increased considerably, and the point of deposition rose from 45° to 48°. The ensuing night the minimum temperature was proportionally raised, the register. thermometer having indicated 50°. The next day, the absolute humidity of the air was reduced, in consequence of an impetuous dry wind, from .282 to .165 grains of moisture in 100 cubic inches of air, and the point of deposition thus descended to 373°. The minimum temperature which followed this very sudden change was 380, differing only three-fourths of a degree from the temperature assigned by the formula. From that day the dryness of the atmosphere gradually increased till the twenty-second, when the quantity of moisture in the air became less than is commonly observed, except in the most rigorous of the winter months. On the evening of that day my formula indicated that the point of deposition was reduced to 26°, and the next morning I had the satisfaction to find that the registerthermometer had, in the course of the night, reached the very same point. Another register-thermometer, which I had placed on the summit of a neighbouring hill, at the height of 905 feet above the point where I usually made my observations, shewed that the greatest cold in that elevated situation had, contrary to what commonly happens, been 1o higher.

This remarkable degree of cold, at that season of the year, seems to have been in a great measure local, and to have extended only a few miles around Amulree. At five o'clock in the morning, the ground in the neighbourhood of that place was so hardened by the frost, that it could not be penetrated by an instrument; and the crops, particularly those of oats and potatoes, suffered severely; the former, which were previously in a greenish state, soon after assuming a pale yellowish appearance, while the latter were completely blackened and withered in the stem. The intensity of the frost was less felt in the neighbourhood of Crieff, which is about ten miles from Amulree, but even there it sensibly affected the standing crops. Around Perth* its influence was scarcely perceptible, the register-thermometer having descended only to 3930; but in the vicinity of that place the air possessed a much greater degree of humidity, the point of deposition, by the application

* Perth is eighteen miles from Amulree, the first range of the Grampians intervening.

of my formula to Dr Gordon's observations at the Manse of Kinfauns, being 38.

The extreme dryness of the air, which preceded the great depression of temperature, during the hight, around Amulree, was not of long duration. On the evening of the twenty-third, the point of deposition rose to 32°, and the minimum temperature of the following night did not descend below 30°. Next day, the quantity of moisture in the atmosphere was more than doubled, so that the point of deposition, assigned by the formula, rose from 32° to 47°. The elevation of the minimum temperature corresponded to this increase of the humidity of the atmosphere, and rose from 30° to 46. These facts seem to establish, in the most satisfactory manner, that a coincidence exists between the lowest temperature of the night and the point of deposition, or that temperature at which the moisture dissolved in the atmosphere would return to the liquid state. The only question with respect to which any difference of opinion can be entertained, is, whether the temperature regulates the humidity, or the humidity the temperature? To this question I have no hesitation in replying, that, from the facts I have adduced, we are warranted in drawing the conclusion, that the temperature, in the first instance, determines the mean hygrometric condition of the atmosphere, at least where the surface of the earth affords a sufficient supply of moisture; and that the latter, in its turn, reacts upon the temperature, and prevents it from being in any case greatly depressed below the point of deposition. In cases where this check operates in a feeble manner, which must always happen when the atmosphere is reduced to great dryness by the translation of air from a higher latitude, or across elevated table-land, the lowest temperature of the night must always be considerable. In the months of April and May, for example, the air over our island is generally extremely dry, on account of the cold and parching northeast winds which prevail at that season, and the minimum temperature of the night is accordingly often reduced below the freezing point. Last season afforded a striking illustration of this. The spring was throughout extremely dry, and the minimum temperature descended almost regularly every night below the freezing point, the only exceptions having occurred when a partial change took place in the humidity of the air. Even on the day of the summer-solstice, though the maximum temperature rose at Perth to 70°, yet the moisture in the atmosphere being so scanty that the point of deposition

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