at which the water, with which it may happen to be charged, ceases to be retained in the gaseous form;-in other words, that dew was nothing more than a very gentle rain, returning at night the moisture evaporated from the earth's surface by the heat of the sun during the day. This view of the subject, as it seemed to account, satisfactorily, for all the appearances connected with dew, was readily received by the ancients; and even in the works of Count Rumford and Mr. Leslie, the most celebrated of modern writers on heat, and the effects of heat on liquid substances, we find the old doctrine distinctly recognized. According to Dr. Wells, however, dew is not produced by the operation of cold upon the atmosphere, generally considered; else would dew, like rain, descend in equal quantities upon all bodies exposed to it; but, on the contrary, he has proved, we think, in the most satisfactory manner, that substances attract dew, with a force which increases in proportion as these substances become, by the radiation of their heat, colder than the atmosphere which surrounds them. From this fact, which has been ascertained by the most varied and careful experiments, it may be stated, as a law of nature, that those bodies which radiate heat most freely, that is, in other words, those bodies, which are denominated bad conductors of heat, are soonest and most copiously wetted with dew.

It was in 1784, that the attention of our author was first drawn to this interesting subject, which, by rather a striking coincidence, appears to have engaged, much about the same time, the thoughts of two other philosophers in this country, namely, Mr. Wilson, of Glasgow, and Mr. Six, of Canterbury. The circumstance which led these three gentlemen to institute experiments relative to Dew, was the fact observed by them all, and particularly by Wilson and Six, that on clear and dewy nights, a thermometer laid on the ground, uniformly indicated a greater degree of cold than one suspended a few feet above it. Dr. Wells and Mr. Wilson were of opinion, that this excess of cold was occasioned by the formation of dew on the surface of the earth, while Mr. Six imagined, that it arose partly from the low temperature of the air through which the dew, already formed in the atmosphere, had descended, and partly from the evaporation of moisture from the ground on which the thermometer was placed. The principal object of the treatise now before us, however, is to prove, that both those opinions were founded in error, and that the cold which accompanies the appearance of dew is not produced by the formation of that fluid, but that the cold precedes the deposition of it, and is, in fact, the cause of that deposition.

Dr. Wells has divided his essay into three parts, entitled, of the

the Phenomena of Dew; of the Theory of Dew; and of several Appearances connected with Dew. It strikes us that this division might be improved; and chiefly with respect to the first and third, as the appearances connected with dew would fall very naturally under the head of the Phenomena of Dew, being, in part, nothing more than the particular circumstances which influence or accompany the production of it. Of these circumstances the most important are the following: It is ne cessary for the deposition of dew that the night be calm and serene; for in cloudy nights the quantity is very small, and in nights which are both cloudy and windy, there is no dew at all deposited. On grass and corn it begins to make its appearance before sun-set, in places shaded from the solar rays; and if no change in the weather occurs, the deposition continues till the sun is again above the horizon. It has been remarked, too, that the quantity formed between midnight and sunrise, is usually greater than that which is formed between sunset and midnight. A good deal depends likewise upon situation. Dr. Wells mentions as a general fact on this head, that whatever diminishes the view of the sky, as seen from the exposed body, occasions a diminution in the quantity of dew deposited on it.

"I placed," says he, "on several clear and still nights, ten grains of wool upon the middle of a painted board 4 feet long, and two feet wide, and one inch thick, elevated four feet above the grass-plat, by means of four slender wooden props of equal height; and at the same time attached, loosely, ten grains of wool to the middle of its underside. The two parcels were, consequently, only an inch asunder, and were equally exposed to the action of the air. Upon one night, however, I found that the upper parcel had gained 14 grains in weight, but the lower only four. On a second night, the quantities of moisture acquired by like parcels of wool, in the same situations as in the first experiment, were 19 and 6 grains; on a third 11 and 2; on a fourth 20 and 4; the smaller quantity being always that which was gained by the wool attached to the lower side of the board. I bent a sheet of pasteboard into the shape of a house-roof, making the angle of flexure 90 degrees, and leaving both ends open. This was placed one evening with its ridge uppermost, upon the same grass-plat, in the direction of the wind, as well as this could be ascertained. I then laid ten grains of wool on the middle of that part of the grass which was sheltered by the roof, and the same quantity on another part of the grass-plat fully exposed to the sky. In the morning, the sheltered wool was found to have increased in weight only two grains, but that which had been exposed to the sky, 16 grains."

VOL, VI. AUGust, 1816.

K

Thus,

Thus, the difference in the quantities of moisture attracted by the exposed and the sheltered wool respectively, was very considerable, and that this difference was not owing to the dew falling from above, in the form of rain, is rendered perfectly clear, from the circumstance that, had the dew descended in that form, the wool under the board in the one experiment, and under the pasteboard-roof in the other, would not have been wetted at all. Besides, as a direct proof of this fact, Dr. Wells placed at the bottom of a hollow cylinder of baked clay, two feet and a half in height, and one foot in diameter, a parcel of wool in contact with the grass; and in this situation, the wool acquired little more than two grains of moisture, while a similar quantity, fully exposed, acquired sixteen grains. Had the dew fallen like rain from the atmosphere, the wool at the bottom of the cylinder would have received as much moisture as the wool laid on the open grass-plat; for, it is almost unnecessary to observe, that, on a night when so much dew was deposited, there would be no wind to interfere with its perpendicular descent. The law explaining these facts will be noticed in its proper place meantime, it may be stated, that it was principally from such results that Dr. Wells was carried to the right theory respecting the deposition of dew.

Again, there is something, it has been discovered, in the mechanical state of bodies, which, all other circumstances being the same, has a considerable influence in modifying the produc tion of dew, and chiefly in regulating its quantity. More of that liquid, for example, is formed upon fine shavings of wood than upon a thick piece of the same substance; and the finer the silk, or cotton, or wool is, which is exposed for attracting moisture from the air, the larger is the quantity imbibed by it.

It is mentioned, as the third general observation, that bright metals, in consequence of some circumstance in their constitution, attract dew much less powerfully than other bodies; and between metals of different kinds there is no small difference in this respect, iron, steel, zinc, and lead, being more easily wetted with dew, than gold, silver, copper, and tin.

Le Roi having asserted that dew is never deposited from the air of cities, Dr. Wells determined to ascertain whether the assertion was founded in truth. With this view, he frequently exposed, at night, ten grains of wool upon a slight wooden frame, placed in such a manner between two ridges of the top of his house, (which he informs us is situated in one of the most crowded districts of London,) as to be three feet distant from the nearest part of the roof. The event was, that upon clear and calm nights dew was always acquired by the wool, though never in any considerable quantity; probably however, as our author

justly

justly remarks, more from the wooden frame being nearly surrounded by buildings much more elevated than itself, than from any particular condition of the air in cities.

One of the most important facts connected with this subject, is the cold which in all cases accompanies the formation of dew. Grass wet with this substance was constantly found to be much colder than the air four feet above it, and this difference of temperature amounted not unfrequently to eight or nine degrees of Fahrenheit's thermometer; and, on some occasions, it was not less than eleven or twelve degrees. This distinction however, was never observable except on calm and serene nights, for if the weather was windy and cloudy, the grass was not colder than the air, no dew being deposited in that state of the atmosphere. Even if the sky becomes cloudy at any period during the night, after the deposition of dew has begun to take place, the temperature of the grass rises nearly to an equality with that of the air above it. Thus, on such a night as that now described, Dr. Wells found that grass, which was at first twelve degrees colder than the air, became in the course of a very short time ten degrees warmer, being only two degrees colder than the lower stratum of atmospheric air.

It follows so naturally from the facts already stated, that it is hardly necessary to observe that the colder any body is, compared with those in the neighbourhood, the greater will be the quantity of dew which it attracts. This law of nature enables us to account for a very familiar circumstance, which from the length of time it has remained unexplained, adds one to the numerous instances on record, of the power of a received theory to blind the eyes even of philosophical observers. We allude to the striking difference, which every one has noticed, between the quantity of dew on grass and that on a common road or a plowed field. On a still and serene night, when dew was forming very copiously on a grass-plat, our author ascertained, by the application of a thermometer, that the grass was sixteen degrees colder than a gravel walk quite contiguous to it, and more than twelve degrees colder than garden mould exposed to the same extent of sky and aspect of the heavens. It is therefore very evident that it is owing to the superior cold of the grass, that it attracts dew so much more plentifully than a dusty road, or a piece of soil newly turned up. We must add, however, what Dr. Wells has very candidly stated, that, although it may be set down as a general result, that when bodies which had been equally exposed to the night air were examined at the same time, those which were most dewed were also the coldest, yet that no such correspondence was found in the experiments performed on different nights, or even in different parts of the same night. Thus during

two particular nights on which grasss was colder than the air, there was little dew; while on the night which afforded the most copious dew ever observed by him, the cold of the grass was only about three or four degrees greater than that of the air. It is rather unfortunate, however, that the Doctor has neglected to mention what was the state of the atmosphere on those occasions; for it is a fact ascertained by himself that the deposition of dew bears a definite proportion to the quantity of moisture which the air holds in a state of vapour, and consequently the scanty dews of the two nights, in which the grass was so much colder than the air, may be accounted for in strict accordance with the fundamental principles of his theory.

Grass is not the only substance which becomes colder than the air, in the absence of the sun. All filamentous and downy substances, such as wool of moderate fineness, very fine raw silk, fine unspun cotton, fine flax and swandown, are susceptible of a great degree of cold; and of these, swandown commonly exhibited the greatest. On a certain night when grass was observed to be fourteen degrees colder than the air, swandown, laid on the ground, was one degree colder than the grass; and this difference of fifteen degrees between the temperature of a body on the surface of the earth, and that of the air a few feet above the earth, is about the greatest Dr. Wells ever observed.

We come now to the "Theory of the Formation of Dew." It has been already mentioned that Aristotle imagined dew to be a species of rain formed in the lower atmosphere, in consequence of its moisture being condensed by the cold of the night into minute drops; which opinion is still entertained by several distinguished writers, and among others by the ingenious Mr. Leslie of Edinburgh, who published in 1813, "A Short Account of Experiments and Instruments depending on the Relations of Air, to Heat and Moisture." Gersten, was the first to call in question this hypothesis; for finding that bodies suspended in the air, a few feet above the ground, were wetted with dew, while similar bodies, lying on the ground, remained perfectly dry, he justly inferred that dew did not descend from the heavens, like rain. Musschenbrock shortly after made the discovery that metals received no dew while other substances attracted it copi ously; from which fact his friend Dufay drew the conclusion that dew is an electrical phenomenon, leaving untouched such bodies as conduct electricity, and appearing upon those which do not transmit that influence. It is enough to observe, in answer to Dufay, that metals are frequently seen wetted with dew, and that charcoal, which next to metals is the best solid conductor of electricity in existence, attracts dew in large quantities. Wilson and Six were very near discovering the true theory of the forma