the last-mentioned philosopher among the substances that are even more productive of cold than wool*. Or it is possible that the breeze may have subsided, and the circumstances of temperature become such as to have allowed the deposition of dew on the paper, but not of its re-formation on the silver. Dr. Wells has also remarked †, that when dew forms upon metals, it "commonly sullies only the lustre of their surface; and that even when it is sufficiently abundant to gather into drops, they are almost always small and distinct." This observation, however, requires some limitation; since, on nights that have been more than usually cold, and when the quantity of moisture in the air has been abundant, I have observed the dewy particles deposited on metals to attain a considerable magnitude; and examples have even occurred of polished tin surfaces being completely covered with thin sheets of water, the result of the junction of the innumerable minute particles deposited on them. On one night, equal squares (their linear edges being one inch. and half) of lead, zinc, brass, copper, and tin, were laid on a large plate of glass, and presented to the influence of a clear sky. At sunrise the next morning, the particles of dew on the different surfaces were found of variable magnitudes; those on the lead being the largest, and of the size represented in fig. 7. Those on the zinc were next in magnitude, as denoted in fig. 8; and the particles on the brass were still smaller, but much more numerous, as in fig. 9. The copper and tin, particularly the latter, seemed only to have had the lustre of their surfaces just dimmed, by the abundant moisture of the air. Lead, therefore, was at one extreme of the series, and tin at the other; brass holding a middle rank between the two. This relation, however, between the particles on lead and brass, was inverted on another night, when equal squares were laid on the recently cut herbage, the particles on the brass being of the size represented in fig. 10, and those on the lead as denoted in fig. 11. As the plates of metal were the same in both cases, it is reasonable ♦ Page 21, Essay on Dew, by Dr. WELLS, Second edition, to infer that the opposite results observed were produced by the substances on which they were respectively placed. A slight trace of moisture was perceptible on the zinc, but not the least degree on the copper and tin. An example of the slowness with which polished tin permits moisture to be deposited on it, occurred when a concave mirror, formed of polished block tin, was employed as an Æthrioscope, on the plan first suggested by Dr. Wollaston. The focus of the instrument, at the time the experiment was performed, was 20 inches above the ground. The night was tranquil, and dew was copiously deposited on glass, a few minutes after it was presented to the chilling influence of the transparent sky. At nine, P. M. the thermometer in the focus of the thrioscope indicated a temperature of 46°; the herbage being at the same time 44°; and the air, seven feet above the ground, 491°. Observations, connected with some other phenomena, were made every half hour; but no trace of moisture was perceptible on the metallic surface, till two a. M., when it appeared slightly dimmed, although other substances had gained considerable increments of dew in the same time; masses of wool, for example, having increased in weight from twelve grains to thirty. At the same moment, the focal thermometer indicated a temperature of 42°; that on the grass, 39°; and that elevated in the air, 45°. In five hours, therefore, the cold of the upper sky only underwent a change of 310; whereas the grass lost by radiation in the same time 5°; and the elevated stratum of air diminished its temperature 41°. From two o'clock to three, the thermometer remained stationary, but the moisture had sensibly increased on the surface of the Ethrioscope, and increments amounting to several grains, were likewise found on other substances; a proof, that if the general temperature remains stationary, after the temperature of a body is sufficiently lowered to permit the formation of dew on its surface, the farther deposition of moisture is not prevented. At four A. M. the whole metallic surface was covered with visible drops, the temperature, at the same moment (just before sunrise), indicating the maximum of cold, the focal thermometer being at 40°; that on the grass, 37°; and that elevated seven feet above the ground, 41°. It is worthy of remark, that two plain sheets of polished tin, placed horizontally on the herbage, had not the slightest trace of moisture on them. On another night, however, when there was every prospect of an abundant deposition of dew, the influence of the grass in promoting its formation on metals, was clearly shown. At nine P. M. two plates of polished tin, one fourteen inches by ten, and the other six by two, were laid on very short grass. Another plate of the same dimensions as the former was placed gently on the long grass. Its weight necessarily compressed the herbage on which it rested, so that the polished surface was surrounded on all sides by grass, reaching twelve inches above it. In fig. 15 the long grass is represented on two opposite sides of the tin M N, together with the compressed herbage below it. At eighteen inches above the ground, or two inches above the average height of the grass, a similar plate, OP, was placed on slender props. The temperature of the grass at the moment the plates were exposed was 60°, and of the air 65°; being a difference of 5° in the small space of three feet. At five the next morning, a great quantity of dew was formed on the grass. A register thermometer on the short herbage, indicated the maximum cold to have been 52°, and of the air, at the elevation before mentioned, 60°. The difference between these maximum depressions of temperature was, therefore, by no means considerable; and the copious deposition of dew observed was to be regarded rather as the result of the abundance of moisture in the atmosphere, than as a consequence of great difference of temperature. The metals presented the following particulars for observation. The plates resting on the short herbage had a few scattered patches of dew on their upper surfaces, but nothing like a regular and uniform deposition. The plate M N, surrounded by the long grass, had its superior surface completely covered with minute but distinct particles of moisture; but the plate O P, elevated above the grass, was perfectly dry. This difference in the results must be regarded as arising from the different conditions, under which the plates were situated. The latter surface, it will appear, had not its tem perature depressed below that of the stratum of air reposing on it, during the night; but the former must have been considerably colder than the column of air hovering above it. The cooling power of the grass surrounding the plate M N, and on which it also rested, must have necessarily extended its influence to the metal; and by lowering its temperature considerably, have occasioned the copious deposition observed. The upper plate not being in contact with the grass, permitted the air to pass freely on each side of it; and being itself a bad radiator, attained no condition during the night favourable to the deposition of dew. With respect to the formation of dew being less abundant on the plates resting on the short herbage, than on that surrounded with the long grass, it may, in one point of view, be regarded as a consequence of the curious fact observed by Mr. Six, that the temperature of short grass is always greater than that of long grass. The state of the herbage has always a considerable influence on the quantity of dew deposited, and the greater the body it presents, the more abundant it is likely will be the formation. That the quantity of herbage has a considerable effect, may be inferred from the experiment, that when one mass of wool was placed on short herbage, and another of equal size and weight on the summit of a mass of recently cut grass, fifty inches above the ground, the moisture gained by the former during the night, was only fifteen grains, whereas the increment to the latter was twenty-three. In consequence of the plate O P having had its surface exposed to the entire canopy of the sky, but the view from the plate M N being confined to a comparatively small circular space, in the zenith of observation, it might be inferred from a principle adopted by Dr. Wells*, that the former would have gained more moisture than the latter. But the maxim of this ingenious philosopher is evidently limited to the consideration, that the bodies are in other respects similarly circumstanced. For instance, in one of the experiments Essay on Dew, page 14, second edition. The principle here alluded to is the following: "Whatever diminishes the view of the sky, as seen from the exposed body, occasions the quantity of dew which is formed upon it, to be less than would have occurred if the exposure to the sky had been complete," instituted by Dr. Wells, to illustrate the principle in question, by bending a sheet of pasteboard into the form of the roof of a house, and placing it with its ridge uppermost, and ends open, over a mass of ten grains of wool laid on the grass; and at the same time placing another equal mass on the herbage, fully exposed to the sky, the former gained, during the night, an increment of only two grains, whereas the latter gained sixteen. In this experiment, the two masses were placed under the same circumstances, so far as contact with the grass was concerned; but in the case relative to the plates of tin, one was not only in contact with the herbage, but also surrounded by it; whereas the other was completely detached. The gradual manner in which dew is deposited on the metallic side of gilded glass was pleasingly exemplified on another occasion. The parallelogram of glass was six inches by four, as represented in fig. 14. It was first exposed to the atmosphere with its metallic side uppermost, at half-past six, P. M., being about three quarters of an hour after sunset. The atmosphere was clear, and highly charged with moisture; and dew had formed on glass in a shady place, three quarters of an hour before the departure of the solar orb. A mild and gentle breeze prevailed also at the same time. No perceptible change took place in the metallic surface until eight, when minute particles of dew were visible at A, the leeward end. From the last-mentioned hour to ten, the moisture gradually increased from A to the middle part of the surface; and distinct drops were likewise deposited at D, B, E, C. As the particles increased in size round the three edges, other minute drops were successively deposited, more distant from them; and it was observed, that they accumulated with most rapidity at the leeward sides A and C. At eleven, P. M., when the sketch represented in the figure was made, an oval portion of the metallic surface was found entirely free from moisture. The same figure was also perfectly visible at midnight, when the drops at A had increased to at least an eighth of an inch in diameter; those at C being rather less. The particles at the corners D and E also preserved their superiority in size above those at B, |