descend far, for the water in its fall would be redissolved by the excess of heat in the lower regions, which might remain transparent and undisturbed. In these different ways, a circulation of temperature would be caused, that would, if unimpeded, speedily restore the equilibrium of heat throughout the sphere. If the diminution of temperature in the higher regions, became great, the evaporation would be enormous, while the condensation would be proportionate, and the precipitation would resemble a waterspout. Such a state of things may, indeed, be considered as affording the probable cause of that hitherto unexplained pheno

menon.

If, instead of ascribing a continuous surface of water to the sphere, we conceive portions of it to be dry, uncovered, and heated to a higher degree than the adjacent seas, constant currents of vapour would, in this case, pass from above the watery surface, to that we have supposed to be dry; the vapour would be heated beyond its original temperature, and might reach its point of deposition at a high elevation, without producing any sensible cloud. The circumstances of the case will be varied, if the uncovered portions of the solid sphere be of such a nature, as themselves to furnish a portion of water by evaporation.

We are to seek for the causes of the retardation of the flow of a current of vapour from the equator to the poles, in the existence of an atmosphere of permanently elastic fluid, surrounding the earth; through this the aqueous matter must filtrate, as water does through sand; and for the cause of the variation of the temperature of the upper regions, from the law by which the heat of an atmosphere composed wholly of vapour ought to decrease, in the difference between the corresponding capacities for heat, in a permanently elastic and a condensible atmosphere, at equal heights. Entering into the investigation, then, of the habits of such a compound atmosphere, we find, that the great aerial currents are not affected by the presence of vapour, but flow on in opposite directions, while the aqueous part is almost wholly confined to the lower of these currents, and presses in a direction contrary to that of the current in which it exists. Thus the compensating winds flow on in the courses we have already described, and the balance of pressure remains undisturbed. We do not, however, yet know the rate at which aqueous matter will travel through a current of air in a direction the opposite of the aerial current; hence these circumstances require farther investigation.

Aware of the importance of the inquiry into the state of the atmosphere, in respect to the vapour it contains, our author undertook the construction of an instrument fitted for the purpose of making observations upon the aqueous matter. In this attempt, he has been completely successful. He states, that he was first

led to the pursuit of the principle he has adopted as the basis of his instrument, by a passage in the works of Pliny. In this it is stated, that when vessels are observed to sweat, or be covered with dew, it is a token of the approach of tempestuous weather. The fact of the deposition of moisture upon vessels colder than the surrounding atmosphere, is one of constant recurrence, and very familiar. If a glass, for instance, containing spring or well water, be exposed to the air on a warm day, it will speedily be covered with a film of condensed vapour. The quantity condensed, will partly depend upon the vapour already existing in the atmosphere, and partly on the difference in sensible heat between the air, and the liquid in the vessel. Instead of spring or well water, water artificially cooled will exhibit the same phenomenon. There are certain salts, that, when dissolved, lower the temperature of the water in which they are placed. If one of these, in fine powder, be gradually added to a portion of water contained in a proper vessel, the temperature may be lowered by degrees, until it reach that point at which deposition will just appear to take place. A thermometer placed in the vessel, will show this temperature, and of course mark the degree of heat at which saturation takes place, with the quantity of vapour at that time present in a given bulk of atmospheric air. This temperature is, as we have already seen, called the dew-point, and by reference to the tables constructed by Dalton from his experiments, it will enable the observer to determine the absolute quantity of moisture then present.

As this experiment would be attended with difficulty in practice, Mr. Daniell has adopted another mode of performing it. If water be placed in one of two balls or bulbs of glass, connected together by a tube bent twice at right angles, and exhausted of air, the immersion of the empty ball in a freezing mixture, will cause the congelation of the water contained in the other. For, the aqueous vapour that rises rapidly in vacuo, will be as rapidly condensed by the cold application; its place will be supplied by a fresh evaporation from the surface, and the formation of this new vapour will speedily carry off so much heat from the mass, as rapidly to reduce its temperature to that of freezing. If we substitute ether for water in either of the balls, and if the ball that contains no liquid be coated with a bibulous substance, moistened also with ether, the evaporation of this last will produce a great degree of cold; and this will not be manifested in a loss of heat by the ball to which the ether is thus externally applied, but by the rapid passage of the enclosed ether in a state of vapour from the other bulb, the temperature of which is in consequence lowered. As soon as the surface of this last bulb is cooled down to the point at which the aqueous matter contained in the atmosphere would be deposited, a thin film of vapour settles on the

bulb, and it becomes clouded; to mark the temperature at which this occurs, a very sensible thermometer is enclosed in this bulb and the contiguous stem. A very little practice will enable an observer to note with precision, the very instant at which this occurs, and to read off at the same moment, the temperature marked by the enclosed thermometer. We know of no instrument more simple and beautiful in principle, than this hygrometer; it is easy and convenient in its use; and it satisfies all the conditions that have been prescribed as essential to the perfection of hygrometric instruments.

One of its most important applications is to the determination of the quantity of evaporation, which is always in a ratio inverse of that in which vapour is present in the atmosphere. It is also valuable, as furnishing an element hitherto unobtainable in the mensuration of heights by the barometer. Local and individual prejudice, has hitherto impeded the introduction of this instrument, but these seem to have ceased to operate. It has been attempted to use a simple thermometer, whose bulb is cooled by the evaporation of ether, in the place of this instrument of Daniell's. This attempt has recently been made by Mr. Thomas Jones, and he has been encouraged in the inquiry by names of no small note, particularly by Kater; and much pains have been taken to prove the plan to be equal, if not superior to that of Daniell. We cannot, however, acquiesce in this, but must still consider the latter as far better suited for the contemplated object.

The phenomena of the radiation of heat from the sun to the earth, and from the earth towards the empty space that every where surrounds it, have hitherto excited but little attention. This, however, cannot be ascribed to their being unimportant, but rather to their having escaped the researches of philosophers. . Both, indeed, have a marked effect upon climate, whether we consider it in regard to its animate inhabitants, or the plants that it is fitted to produce. Our author has entered more fully into this inquiry than any of his predecessors; indeed, we believe, that we cannot name any authors who have added much to our stock of knowledge on this head, with the exception of a slight, but important hint, thrown out by Dr. Young, in relation to the formation of dew, and the extension and full illustration of the same idea by Dr. Wells. The latter, however, it is but just to state, was probably ignorant of the step made by the former.

In his inquiry into the solar radiation, our author has been assisted most materially by Captain Sabine, who, in the course of his voyages, has made numerous and accurate observations on the comparative heat of a thermometer acted upon solely by the air, and of one exposed to the direct rays of the sun. In these

observations, the bulb of the exposed thermometer must be blackened, in order that all the calorific rays may be absorbed ; and it is evident, that the difference between it and one suspended in the shade, will be the measure of the sun's radiation. The result of this investigation is remarkable: in a given latitude, the radiation does not follow the law of the mean daily temperature of the atmosphere, but that of the sun's declination; being greatest in the northern hemisphere in June, and least in December; in high latitudes, the radiation is much greater than in the equatorial regions, at equal elevations above the sea; for instance, Captain Sabine found the difference between a thermometer covered with black wool, and exposed to the sun, and one in the shade, to be in Jamaica, no more than 18°3' at a maximum, while at Mellville Island, it was found to amount to 55°; in high regions of the atmosphere, the solar radiation is much greater than near the level of the sea; for, the same observator, found the difference of the two thermometers to be, in the Island of Jamaica, at the height of 4000 feet, as much as 57°, while on the seashore, it was only 36°. As a confirmation of the second of these deductions, our author quotes the authority of Captain Scoresby, who remarked, that the rays of the sun, falling in the arctic regions upon the blackened sides of a ship, occasionally melted the pitch, while on the other side of the vessel, and sheltered from the sun, ice was rapidly forming; and that on the one side, a thermometer would rise to 80°, 90°, or even more, while it stood at 20° on the other. The last of the inferences, is confirmed by an extract from the Journal of the celebrated and accurate Saussure, who also found the heating power of the sun greatest at high elevations. We conceive that Mr. Daniell has been successful in pointing out the cause of these phenomena. In consequence of the unequal distribution of heat in the sphere, the depth of the atmosphere is much greater at the poles than at the equator; so much so indeed, that this difference almost compensates for the greater inclination of the solar rays in high latitudes; the rays are therefore compelled to traverse a greater extent of aerial medium, before they reach the earth in the lower latitude. Now the air is not absolutely transparent, but reflects and absorbs a portion of the light transmitted through it, and this absorption will in part depend upon the distance the rays traverse. The inequality of density does not counteract this absorption, but in truth increases it; for the cooling power of the air, arising from its capacity for specific heat, increases, with its elasticity. As this elasticity and increased depth of the atmosphere arise from the very action of the sun, he thus sets bounds to the intensity of his own influence; and by this admirable adjustment, the force of solar radiation is tempered in those regions, where its full action would be destructive to both animal and ve

getable life, and is more developed in climates, where a greater intensity is necessary to counterbalance the obliquity of his rays. In this way, an additional stimulus is given to the vegetation of the polar regions, where the rapidity with which verdure springs has been often remarked, and is far beyond any thing of the kind in more temperate climates; plants spring up, bear their flowers, and produce seed, within the space of a month or six weeks.

The radiation from the surface of the earth throws off, as we have seen, a quantity of heat that is, for any long period, exactly equal to that received by radiation from the sun. But this equality does not subsist in the changes of day and night, and in the vicissitudes of the seasons. The terrestrial radiation varies only with the heat of the earth, while the solar is entirely intermitted during the night, varies with every hour of the natural day, and is different with every change of the sun's declination. This constant variation in the one, and near approach to constancy in the other, are the causes of several of the phenomena of climate. Thus when the sun sinks below the horizon, at any given place, the surface, no longer receiving any heat, but continuing to radiate, cools rapidly; and will, even in the hottest weather, assume a temperature beneath the existing dew-point of the atmosphere. A deposit of moisture will therefore take place upon the surface; this precipitation, if the air be still, will be propagated upwards, and a mist will be formed, which will appear to rise from the earth, while the aqueous matter which composes the cloud, really descends. In this way, Dr. Wells accounted for the formation of dew; and his explanation of this phenomenon, is now universally received. Surfaces that radiate well, moist earth, shallow lakes, and morasses, will cause the greatest precipitation, and be covered with the greatest quantity of dew; as loose earth radiates better than when it is hard and compact, tillage increases the deposit, and adds to the fertility of the soil. In a cloudy state of the atmosphere, the radiation of heat is interrupted, and the formation of dew lessened, so that its absence may be considered as an almost certain forerunner of rain. High winds also, by preventing any portion of air from remaining in contact with the same part of the surface, prevent this precipitation. The atmosphere itself, does not cool nearly as rapidly as the earth; hence dew will be formed, when the temperature, a few feet from the ground, is far above the dew-point. When the radiation cools the surface below 32°, the dew is frozen, and forms what is called hoar frost; and this may occur at times, when the heat of every part of the ground, (except its mere surface,) and that of the atmosphere, are considerably above the temperature of freezing. In the northern parts of the United States, such is the intensity of this cause, that, in damp situations, sheltered from the wind, frosts occasionally occur in almost