s the humidity which the air, under certain circumstances, deposits, in the form of minute globules, on the surfaces of the bodies in contact with it. The Greek term ἀνάστασις was evidently derived from ἀνάστασις, aqua, implying simply watering or humidification. The Latin name ros is of the same descent. Our English word is obviously borrowed from the German thau, akin to the verb which signifies to melt, and conveying the idea therefore, in the Shaksperian phraseology, of air "melting, thawing, and resolving itself into a dew." The Swedish term dag is no doubt of the same origin, though it likewise denotes low mist or floating vapour. It is remarkable that the French language, though certainly not remarkable for its copiousness, has two distinct terms for dew: serine for the humidity which collects in the evening; and rosée, for what appears accumulated in the morning; the latter being derived from the Latin word ros, and the former intimating that clearness and serenity of the sky which is most conducive to the formation of dew.
When the atmosphere has a temperature below the point of congelation, the dew which might adhere to the substances exposed to it passes into the form of hoar-frost. This was called by the Greeks τρύγον, from its hard or consolidated nature. The French term is exactly the same compound as our own, white frost or gelée blanche. But the German language has a simple and primitive word to denote it, rief, which, in the Swedish, has been slightly modified into rim, a word likewise adopted by the older English writers, and still retained in the Scottish dialect, or dilated into rime-frost, and thence probably corrupted into raw-frost.
As dew appears to collect only during fine clear nights, when the heavens glow with sparkling constellations, the ancients, in the infancy of science, imagined it to be actually shed from the stars, and, therefore, to partake of a pure and celestial essence. Hence the vulgar notion that dew falls, which has prevailed through all ages, and continues to tincture every language. The mythologists described dew as the daughter of Jove and of the Moon, and Plutarch asserts it to be most abundant in the time of full moon. The lunar beams themselves were supposed to contribute some influence, being of a cold nature, and therefore possessed of a humidifying quality. The moon, it was imagined, performed merely the office of an imperfect mirror, reflecting the softened lustre of the sun without any portion of his heat.
The dew of heaven has always been regarded as a fluid of the purest and most translucent nature. Hence it was celebrated for that abstenent property which, according to the vulgar persuasion, enables it to remove all spots and stains, and to impart to the skin the bloom and freshness of virgin beauty. Like the elixir of later times, it was conceived to possess the power of extending the duration of human life; and Ammianus Marcellinus ascribes the longevity and robust health of mountaineers, in comparison with the inhabitants of the plains, chiefly to the frequent aspersion of dew on their geld bodies. Dew was also employed as a most powerful agent, in all their operations, by the alchemists; some of whom pretended that it possessed such a subtle and penetrating efficacy as to be capable of dissolving gold itself. Following out the same idea, the people of remote antiquity fancied that the external application of dew had some virtue in correcting any disposition to corpulence. The ladies of those days, anxious to preserve their fine forms, procured this celestial wash, by exposing clothes or fleeces of wool to the humification of the night. It was likewise imagined that grasshoppers feed wholly on dew, and owe their lean features perhaps to such spare diet.
The philosophers of Greece, after genuine knowledge had illumined that interesting region, entertained far juster notions concerning the nature and formation of dew. Aristotle, whose universal genius ranged over both the physical and the intellectual world, studied facts closely, and sought to reason accurately from the phenomena actually observed. In his book De Mundo, he defines "dew to be humidity detached in minute particles from the clear chill atmosphere." In his treatise of Meteorology, he states that "dew is only formed beneath a calm and cloudless sky, but never in windy weather." He further subjoins, that it collects in low places, and not on the summits of mountains. Vapour, which, according to him, is only heat combined with water, rises in the atmosphere during the day; but when the cold begins to prevail at night it again discharges its humidity. This vapour, however, he thinks can never ascend high above the surface of the earth, both because it must soon lose its buoyant heat, and because in lofty situations it would be scattered and dissolved by the agitation of the air. Dew is hence most copious in fine weather, and in low damp situations. A north wind checks its production, but a gentle southern gale, charged with humidity, will occasion a copious deposit. When a more intense cold prevails in the atmosphere, the vapour precipitates its humidity in a congealed form, and the dew passes immediately into hoar-frost. Cold occasions this consolidation. Dew has hence the same relation to hoar-frost that rain bears to snow, the frozen mass of clouds constituting the one, and attenuated low vapour, seized by frost, forming the other. The heat of the sun's rays thus first raises the vapour from below; but, in all its subsequent changes and modifications, the moon and stars, contrary to the earlier and more popular notions, exert no sort of influence.
The Aristotelian opinions seem to have given place among the Romans to the ruder notions which prevailed among remote antiquity respecting the mode of its formation. Pliny invariably speaks of a dew as falling from the heavens; cum ros cecidisset. We might expect, therefore, that the poets would continue in their verses to perpetuate the same idea.
Sparsaque celesti rare madebit humus. OVID, Fast. i. 312.
Vitreoque madentia rogo. Tempora noctis eunt. Id. Fast. iii. 890.
Qua prata jacent, Quae roriferam ulcens aura, Zephyrus vernas evocat herbas. SENECa Trag.
Hinc ubi roriferis terram nex obruit umbrae. LUCRET. vi. 4. 64.
Virgil marks the cold which always accompanies the formation of dew, and which, when it becomes more intense, converts the lucid globules into spicular shoots of hoar-frost.
Cum primum gelidos rores aurora remittit. Eclog. viii. 15.
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1 Gesner, Faciolatus, and other lexicographers, blend this idea in their definition of dew. Ros, Humor celo diffusus nocte, cum sundum est, et convale aura genicat; qui si gelo crescat, est PRIMA. 2 Dux fugiens non eripit nisi eum luna. 3 Status has the expression roriferam luna. 4 Rore alatur clauda. PLIN. xi. 26. Dumque thymos pascuntur apes, dum rore clauda. VIRG. Eclog. 5 Dux roriferis terram nex obruit umbrae. 6 The same observation is repeated by Pliny, Nat. Hist. xviii. 20.—Neque in modo sequi se flatus eundant rores; and in another place, still more explicitly, thus: Rores neque gelo, neque ardoribus, neque ventis, nec nisi serena nocte. Id. ii. 70. The opinion that dew falls from the sky maintained its credit during the course of the middle ages. The alchemists even carried this idea so far as to fancy that, since the dew gradually evanishes in the progress of the day under the action of the solar rays, it then merely seeks by sympathy to regain its native seat in the highest heavens. Nay, some of those ingenious enthusiasts have not scrupled, in confirmation of their wild hypothesis, broadly to assert that a few drops of morning dew, being inclosed in an empty egg-shell, which is placed at the foot of a ladder resting against the roof of a house, the shell will become buoyant while the sun shines, and will mount along the ladder till it reaches the very top. The famous Van Helmont, who refined on the notions of the alchemists, considered the lights of heaven as of two distinct kinds, the one which flows from the sun and rules the day, being intrinsically warm, and possessing masculine virtue; the other, which rules the night, and emanates from the moon and stars, being of a feminine nature, and having a cool or refrigerating influence. This cold light, he imagined, produces the purest essence of water, which is stored in the moon, to recruit the waste of the nether world; and he supposed the allegory of Jacob's ladder might represent that perpetual ascent and descent of aqueous matter, by which the revolution of the system is constantly maintained. That the moon's beams are naturally cold, he thought sufficiently established by the prevailing belief of the common people, who carefully avoid sleeping in the open air, without some screen to protect them from the chilling impressions which are shot down upon the ground.
Baptista Porta asserted that air is actually converted into water from the accession of cold, and thought this transmutation proved by the fact that, on the approach of severe weather, the windows of an apartment have the inside of the panes of glass covered with moisture. Gaspar Schott, as late nearly as the middle of the seventeenth century, was so much persuaded of the coldness of the moon's rays, that he stoutly appealed to the effects of their concentration in the focus of a reflector. This experiment, however, was made sixty years before, and with an opposite result, by the famous Sanctorio, the founder of scientific medicine, and the inventor of the thermometer. He actually employed the air thermometer for the first time; but this very ingenious inquirer must have been deceived by some extraneous circumstances, when he saw the liquor sink so considerably as he asserts, under the calorific action of the moon's light. The philosophical ideas of Sanctorio were perhaps in some instances too refined for his age, and the vulgar notion concerning the production of dew continued afterwards, for more than a hundred years, to be still generally retained. But the progress of horticulture near the latter part of the seventeenth century brought out some unexpected facts, which seemed at variance with the popular belief. It was remarked that a bell-glass being placed in the evening over a plant, was in the morning profusely covered with dew on the inside, though scarcely any moisture appeared to adhere to the external surface. The humidity which formed the minute globules must therefore have risen from the plant or the ground, and adhered against the glass.
Such, however, was the very slow advance of sound philosophy, that Perlicius, who proposed what he calls a Drososcope, consisting of an oblique balance playing in a soft rack, for indicating the quantity of dew accumulated in the absence of the observer, concludes the discourse which, under the direction of Professor Weidler, he delivered in scholastic form on taking out his degree at Wurtemberg in 1727, with the general inference that dew descends to us from the atmosphere of Jupiter, Venus, the Moon, Mars, and Saturn; but that, though it falls from the air, it by no means originates in this fluid. He had found in the month of August that 250 grains of dew formed on the surface of a square foot in the country, while only ninety-four, and at other times seventy-six, or even sixty-four grains, were deposited in the town.
The first person, however, who appears to have openly rejected the invertebrate opinion of the descent of dew, was Gersten, another German professor, who made several experiments on the subject, and printed at Giessen, in 1733, an academical thesis, in which he advanced the opposite hypothesis. He found that all plants exhale, in various proportions, the moisture which forms the aqueous deposit; and remarked, that plates of copper exposed during the night have only their under surface bedewed. This dissertation led the celebrated Professor Muschenbroeck to repeat the same observations at Utrecht. Having obtained similar results, he communicated the main facts to his Parisian correspondent, M. du Fay, who planned immediately a series of experiments on a large scale. This ingenious philosopher caused two tall ladders to be set up, reclining against each other, in a vacant space, remote from all trees and lofty buildings; and on the 25th October 1736, at four o'clock in the evening, the weather being clear and calm, he laid panes of glass on the steps at the different heights of six, thirteen, seventeen, twenty-five, and thirty-one feet above the ground. These he visited at certain intervals during the progress of the night. By five o'clock a pane close to the ground had its under side completely wet, while its upper side was only slightly dewed. At six o'clock the pane, six feet above the surface, was covered with dew; and at seven o'clock the effect had reached the highest pane. During the whole night the dew continued to form; but it appeared always more copious on the lower panes.
These facts might be deemed sufficiently conclusive; but M. du Fay sought likewise to ascertain the relative quantities of moisture deposited at different altitudes. He procured several rectangular bits of green cloth, cut six inches long and four inches broad, and adjusted all to the same weight. These he suspended in horizontal positions at four o'clock in the evening, one of them only half a foot above the ground, and the rest at the heights of six, thirteen, and twenty-five feet. On weighing them next morning he found that they had respectively imbibed fifty-three, sixty-six, fifty-six, and fifty-four grains of dew. The subsequent night having proved windy, they gained only seven, nine, ten, and six grains. It was evident, therefore, that dew is formed not only sooner, but more copiously, near the surface of the ground than at greater elevations.
To determine still more precisely the several quantities of moisture imbibed at different heights, M. du Fay took three linen towels, each three and a half feet long and two and a half broad, and having dried them thoroughly in the sun, he stretched them horizontally at one foot, and seventeen, and twenty-eight feet above the ground. After exposure during the whole night, the air being quite clear and calm, the lowest one was found to have gained 3060 grains, the next only 2346, and the highest 2742. There occurs some slight anomaly in this result; but on reducing the greatest effect to English measures, it corresponds almost exactly to a cubic inch of dew for each square foot of the surface. This might appear to be rather a low estimate for the climate of Paris in the autumnal season, since, at the same rate, it would give only a deposit of about two and a half inches during the whole year.
In the mean time, Muschenbroeck made observations on Discovery the humification of substances placed above the leaden platform of his observatory at Utrecht. The dew formed in such a situation, he thought, could not have risen from the ground, but must have fallen from the atmosphere. But, pushing these experiments further, he was conducted to a most curious and interesting discovery. He found that dew forms in very different proportions on different bodies, and that it will scarcely adhere to a surface of polished metal, while it streams profusely over glass or porcelain. Even the colour of the substance appeared, in some instances, to alter the effect. Thus, a piece of red morocco leather acquired, by exposure through the night, twice as much dew as another piece of the same size, whether black or blue. A closer examination, however, convinced him that this modification was not caused by the mere colour itself, but was occasioned by the addition or infusion of the matter employed to produce it.
M. du Fay, on his part, repeated those experiments by Du Fay with the same success. He likewise performed others of a similar description. Electricity was at this period in high vogue, and he had distinguished himself in that department. It, therefore, struck M. du Fay that the disposition of certain bodies to attract or to repel dew was somehow connected with the distinction into electrics and conductors. In the prosecution of this idea he sought to compare the humifying action of vitreous with that of resinous substances. He took a tin basin, of the same shape and dimensions as one of glass, and having coated it on both sides with forty or fifty applications of a solution of shell-lac in alcohol, he exposed the two vessels during a fine clear night; but the surface of the glass was found to gather twice as much dew as the resinous coating. In the course of his researches, he likewise noticed another curious fact, the explication of which must be referred to a recent discovery. Having selected two large and equal watch-glasses, he set their convex faces horizontally, the one on a porcelain and the other on a silver saucer, and exposed them both, in this position, to the influence of the night-air; when the former was always observed to collect five or six times more dew than the latter. The metallic surface must have, therefore, in some way or other, contributed by its presence to check the precipitation of humidity, though its direct action is confined to a very narrow limit, since, under a resinous coating of only the hundredth part of an inch in thickness, the effect is precisely the same as with a cake of wax. In confirmation of this remark, having composed a square plate, by cementing at the edge a polished rectangle of brass, six inches long and three inches broad, to a similar piece of glass, he found, on exposing it to the atmosphere at night, the vitreous surface covered as usual with dew, while the metallic one was scarcely at all affected; but, on laying another slip of glass across these plates, the end which rested on the brass remained quite dry, while the other end soon became profusely wetted.
It seemed to follow from the experiments of Muschenbroeck and Du Fay, that, strictly speaking, dew neither falls nor rises, but, according to the doctrine of Aristotle, only separates, under a certain change of circumstances, from the air, and attaches itself to some substances in preference to others. The theory of vapour, proposed afterwards by Le Roy of Montpellier, threw further light on the subject. Moisture is suspended in the atmosphere by a real chemical solution, in the same manner as nitre and other salts are dissolved in water. The solvent energy is in both cases augmented by the addition of heat. A rise of temperature enables the air to support a larger portion of humidity, while the decrease of it enfeebles the attractive power, and occasions a precipitation in the shape of mist or dew. This perspicuous explication, as we have seen, had been already anticipated, though but vaguely stated, by Aristotle.
A deposition entirely similar to dew or hoar-frost is hence formed, whenever the air becomes suddenly chilled, by touching any surface much colder than itself; and not consisting of polished metal. Thus the walls of long passages, vaults, or massive buildings, generally drip with wet during the early part of the summer, before the external heat has sufficiently penetrated. In like manner, it is observed, that when a severe and long-continued frost is succeeded by a thaw, the backs of houses are quickly incrusted with shoots of hoary icicles.
The formation of dew is hence easily produced artificially in a close room, without waiting for exposure under a clear nocturnal sky. If a carafe, filled with water from a spring or well, be carried into a warm apartment, the outside of the glass will become soon covered over with an aqueous deposit, which must increase till the body of water has acquired nearly the temperature of the encircling air, and will afterwards gradually disappear. But if a piece of tin-foil be applied to the bottle, it will remain dry, while the rest of the surface appears humidified with dew.
It is a curious fact, that air always begins to deposit upon its moisture on glass, even before it has reached the point of saturation, or become absolutely damp. This property, and the circumstances connected with it, Professor Leslie discovered, in the year 1798, by help of his hygrometer, which he had already brought nearly to a state of perfection. On exposing wine-glasses at the approach of evening, they were soon covered with dew, while this instrument still indicated several degrees of dryness. The difference was yet greater in summer than in winter. The hygrometer being inclosed within a small glass receiver, placed on a wetted plate likewise of glass, stood, after the whole internal surface had become lined with dew, at 5° in a very cold room, but at 15° when the apartment was kept warm.
The general observation is explained by Professor Leslie's researches into the propagation of heat. No adjoining substances can ever come into absolute contact; but air approaches much nearer to the boundary of glass, porcelain, or paper, than to a surface of polished metal. By an extension, therefore, of the principle of capillary action, the suspended aqueous particles, which have a strong adhesion to glass, to which they are brought so close, readily detach themselves from their union with the air. But the same particles being held back from the proximity of a metallic surface to which they have little attraction, are never deposited on it unless the air is actually overloaded with them. The hygrometer will accordingly reach the absolute zero when it is shut up within a case of polished tin.
Professor Leslie afterwards made several occasional observations relative to the production of dew, particularly during a series of clear warm days in the month of May 1801, at Eastbury, in Dorsetshire, where he placed his instruments both on the lawn and on the balustrade of a tower sixty feet in height. Without entering into details, it may be sufficient here to mention the principal results. The periodical variations, both of the hygrometer and thermometer, were much greater near the surface than at some elevation. On the approach of sunset the thermometer on the ground sunk rapidly; the hygrometer relapsed to about five degrees, and the dew began to form on the blades of grass. During the night, the thermometer descended still lower, the hygrometer indicated absolute humidity, and the lawn was covered with a profusion of dew. But a little after sunrise, the thermometer again mounted, the hygrometer began to act, and the sheet of moisture gradually exhaled. In the progress of the day the heat and dryness increased; and, about two o'clock, the thermometer and hygrometer, both of them screened from the direct action of the sun, stood generally, the former at 75°, and the latter at 85°. But, on the top of the tower, all those changes were less violent. The thermometer, which at that altitude seldom rose in the course of the day to 70°, or the hygrometer to 63°, indicated, as night again closed, a depression, though very moderate in comparison with what was shown at the surface of the ground. The thermometer stood several degrees higher than below, the hygrometer remained at a dryness of 15° or 20°, and no dew was deposited on the balustrade. But similar differences of effect, although on a smaller scale, were exhibited at very moderate heights. On lifting those instruments in the evening only a foot from the ground, the thermometer would rise a degree or two, and the hygrometer mount from the verge of moisture to perhaps 10°. At an elevation of four feet those changes were nearly doubled. The dew thus always began, as in Du Fay's experiments, to form at the surface of the earth, and continued to mount upwards with the progress of the night.
It is hence easy to explain the general phenomena of the dew. "In fine calm weather, after the rays of the declining sun have ceased to warm the surface of the ground, the descent of the higher mass of air gradually chills the undermost stratum, and disposes it to dampness, till their continued intermixture produces a fog, or low cloud. Such fogs are, towards the evening, often observed gathering in narrow vales, or along the course of sluggish rivers, and generally hovering within a few inches of the surface. But in all situations, these watery deposits, either to a greater or a less degree, occur in the same disposition of the atmosphere. The minute suspended globules, attaching themselves to the projecting points of the herbage, form dew in mild weather, or shoot into hoar-frost when cold predominates. They collect most readily on glass, but seem to be repelled by a bright surface of metal." The unequal heating of the surface during the day thus occasions, on statical principles, a perpetual interchange between the higher and the lower atmosphere, which is prolonged through the night, the warm portions of air still continuing to ascend, and leaving their place to be occupied by the descent of similar cold portions of that fluid. This vertical play is a provision of nature for the tempering of the diurnal vicissitudes of climate. "In clear and calm weather, the air is always drier near the surface during the day than at a certain height above the ground, but it becomes damper on the approach of evening; while, at some elevation, it retains a moderate degree of dryness through the whole of the night. If the sky be clouded, less alteration is betrayed in the state of the air, both during the progress of the day and at different distances from the ground; and if wind prevail, the lower strata of the atmosphere, thus agitated and intermingled, will be reduced to a still nearer equality of condition."
The descent of chill air caused by superior density, explains the formation of dew in low situations, and its progressive elevation as the cold accumulates. But some further explication was wanted to reconcile the concluding observations of M. du Fay. The subject was in consequence resumed by M. Benedict Prevost, who performed a curious set of experiments, described in a Memoir read before the Philosophical Society of Montauban in 1803. The results are certainly perplexing, and would almost seem anomalous. 1. Tin or copper foil, and gold or silver leaf, being applied to plates of glass, and exposed to dew, were observed, as before, to remain generally dry, while the vitreous surface became bathed with moisture. 2. After exposure to the night air, not only a dry border appeared, extending a little way beyond the film of metal, but the side opposite to that coating continued still dry, though all the rest of the glass was profusely wetted.
3. A piece of glass, being laid above the metallic leaf, destroyed its effect. 4. A rectangular piece of tin-foil being pasted on the inside, at the top of a pane of glass, in a window having a northern exposure, and a similar piece applied at the bottom on the outside; when the dewing began first against the inside, the interior coating appeared wetter than the naked surface, and the portion of this immediately behind the exterior coating seemed always drier than the rest. The facts were exactly reversed when the dewing commenced on the outside of the window. 5. Opposite to the middle of a rectangular leaf of metal, a similar but smaller piece being applied on the outside of the pane, when the dew began to form within the apartment, the space behind the exterior coating still remained dry. 6. In all cases, whether on the inside or the outside of the pane, on covering the metallic leaf with a piece of glass of the same dimensions, the effect was exactly the same as if no metal had been interposed. 7. Similar appearances were produced by combining gilt paper or quicksilvered glass, the results depending wholly on the nature of the extreme surfaces, according as they consisted of metal, or of glass or paper.
On reviewing these curious facts, M. Prevost was struck with their apparent analogy to the phenomena of electricity. He thought they might all be comprised under a single proposition: That glass which separates two masses of air of unequal temperatures attracts or repels humidity according as it is armed with metal on the hot or on the cold side. To account for these very singular yet interesting facts, he proposed a random and strained hypothesis, grounded on some loose notions of chemical affinities. But we need not stop to examine it.
Dr Thomas Young, in his Lectures on Natural Philosophy, published in 1806, concludes a short abstract of the excellent views of Prevost with suggesting that they would derive their explication from Professor Leslie's Discoveries on Heat. The anticipation was perfectly just, though the discoveries themselves required then a little further extension to embrace the whole phenomena. Professor Leslie had carefully investigated the laws which modify the propagation of hot or cold pulses through an aerial medium from a solid or a liquid boundary. But he did not contemplate the pulsation excited at the conterminous surface of two strata of air having different temperatures. It was indeed impossible to devise an experiment in which the opposite layers of fluid could be kept distinct, for the warmer portions of air would seek always to rise, while its colder and denser portions would endeavour to sink downwards, and thus form, by insensible shades, a vertical gradation of temperature. But though the pulsatory action excited at each successive horizontal stratum might singly escape observation, it seemed probable that the accumulated impressions transmitted from numerous boundaries would become very sensible. Accordingly, in a close heated room, the pyroscopé, or differential thermometer, having one of its balls gilt, which is susceptible of such pulses only, marks, near the floor, perhaps four or five degrees of calorific impression, yet, when lifted higher, it indicates an effect always diminishing in proportion to the proximity of the ceiling. The entire action exerted, or the amount of the intermediate energies, was therefore, as the excess of the temperature of the stratum of air next the ceiling above that of the stratum in which the instrument happened to be placed. Carried out of doors in clear and
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1 Leslie On the Relations of Air to Heat and Moisture, p. 132. 2 Ibid., p. 132. calm weather, after the sun had withdrawn his beams, it betrayed a much stronger tendency the contrary way, and marked a copious frigorific impression, evidently produced by the coldness which must pervade the upper regions of the atmosphere. But to fit the pyroscope for making observations during the day, it was converted into the *Athrioscope*, in which the influence of light is neutralized; a combination of great delicacy, and, therefore, a valuable acquisition to meteorological science. (See the article *Climate.*)
The application of this new instrument has not only ascertained the existence, but measured the intensity, of the cold pulses which are at all times darted downwards from the successive strata of air, though often partially intercepted by clouds, or more completely obstructed by low fogs. But since the spheroidal cup, which concentrates the various oblique impressions on the upper ball of the athrioscope, can do little more than double the direct action against a horizontal surface, it may hence be computed that, in fine bright evenings, those cold pulses rained from the sky are sufficient alone to depress the temperature of the ground, according to the seasons, sometimes eight degrees, but generally about three degrees, by Fahrenheit's scale. The blades of grass, thus chilled from exposure, cool in their turn the damp air which touches them, and cause it to drop its moisture.
For the same reason, the naked ball of the athrioscope, as it is still more cooled, appears much sooner affected, being commonly covered with profuse liquid globules long before the dew has begun to form on the surface of the ground.
All the difficulties and seeming anomalies of the observations of Du Fay and Prevost now vanish away. The various phenomena proceed chiefly from the cold induced by exposure under a clear sky; but other causes will often essentially modify the results.
1. The impression received on a plate of polished metal scarcely amounts to the tenth part of what is communicated to a surface of glass, wood, cloth, paper, earth, or grass.
2. When the action continues the same, the corresponding depression of temperature yet depends on the slowness with which the cold is subsequently dispersed. In calm weather a plate of glass, or a sheet of paper, if covered on both sides with a leaf of metal, will gain or lose heat twice as slow as before, and if coated only on one side, its progress will be a half slower.
But high winds greatly assist the dispersion of heat, and often reduce the effects of external impressions to the third or the fourth part of their ordinary measure.
Hence the reason why scarcely any dew is formed in windy weather, though the sky be clear; for the frigorific pulses must then have little efficacy, not cooling the ground perhaps more than one or two degrees. In the last observation of Du Fay, the slip of glass laid across the rectangle, composed of alternate bars of glass and of brass, being greatly chilled by exposure, had by contact communicated its coldness to the matter under it, and thus enabled the metal to assist in the deposition of dew. In Prevost's second experiment, the metallic leaf, being scarcely affected by the frigorific impressions, checked by its presence the progress of cold along the vitreous surface, and therefore maintained a dry border all around it. Hence, in his third experiment, a piece of glass covering the metal received the entire impressions, and restored the former effect. The application of a metallic coating against the inside of a pane, must, in the fourth experiment, have augmented by one half the efficacy of the external pulses of cold, and thus made the dew to attach more profusely. For a like reason, while a leaf of metal on the outside of a pane became, in the fifth experiment, slightly dewed, the addition of a smaller metallic leaf against the inside increased the effect, by promoting the accumulation of cold.
The remarkable experiments which the late Dr Patrick Wilson, professor of practical astronomy in the university of Glasgow, performed during the severe frost of January 1780, are easily explained on the same principles. In the declivity of a garden, a thermometer laid, in a clear starless night, on the surface of the snow, stood from eight to ten degrees lower than when suspended at the height of a few inches. This excessive cold was evidently not occasioned by evaporation; for, on blowing with bellows against the bulb when it lay on the snow, so far from sinking more, the mercury actually rose two degrees higher than its station in the free air. The intensity was no doubt in part owing to the low position of the snow; for a thermometer suspended at a pole projecting from a window twenty-four feet above the surface indicated four degrees less cold than below. But, besides the accumulating action of the descent of cold air, the snow must have been also chilled extremely by the frigorific pulses darted from an azure sky. This inference, though not perceived at the time, or, indeed, likely to have been admitted then as philosophical, is distinctly supported by an experiment of Dr Wilson. Having screened a spot of the garden by a sort of sharp roof formed with two inclined sheets of brown paper, and laid a thermometer under it on the surface of the snow, the instrument soon marked six degrees of less cold than before, or than another exposed at only a short distance. But this open screen, since it could not impede the mere descent and influx of cold air, must have intercepted a more powerful frigorific influence.
Dr Wilson afterwards performed other similar experiments, which are detailed in his paper on hoar-frost, drawn up in 1788, and inserted in the first volume of the *Transactions of the Royal Society of Edinburgh*. He made the important remark, that during a fog there was no difference of temperature between the surface of snow and the incumbent air. But he neglected to pursue the consequences, and was disposed, from the various facts which he had observed, to conclude vaguely that hoar-frost is always accompanied by a production of cold.
About the same time Mr Six of Canterbury, the inventor of the self-registering thermometer, employed that very useful instrument in making similar but more extensive observations. He found, in a clear summer evening, his thermometer, when laid on the grass, to sink five degrees lower than when suspended freely near the surface. But he had occasion afterwards to remark still greater differences. On a clear and still night in winter, the thermometer which had been supported in the air fell no fewer than thirteen and a half degrees when placed flat on a meadow. He likewise noticed, as Dr Wilson had done, that thick fogs always impede, and often wholly prevent, the peculiar cooling of the ground.
It seemed therefore ascertained that, in the absence of the sun, the surface of the earth, and especially its projecting herbage, acquire, in calm weather, from the mere aspect of a bright and unclouded sky, a very notable degree of cold. This cold appears likewise connected evidently with the formation of dew. But what is the nature of that relation? Is the coldness contracted by substances on exposure to the nocturnal air to be considered as the effect or as the cause of their dewing? The former opinion, we have seen, was espoused by Dr Wilson, though sound theory should make us expect that the deposition of dew, or the conversion of humidity from a gaseous to the liquid state, must, on the contrary, occasion a small extraction of heat. But constant experience shows that cold bodies, not sheathed with metallic lustre, become always sprinkled with minute aqueous globules, from the contact of damp air. The simplest truths, however, are very seldom the soonest perceived, and the late ingenious and learned Dr Wells has the merit of being the first who distinctly attributed the formation of dew to the previous cold induced on the ground from the aspect of the sky. He had early conceived an opposite idea, but a closer examination of the subject led him to adopt juster views. Being once engaged in the research, he prosecuted his observations with assiduity and ardour for upwards of two years, at a friend's villa on the skirts of London, in spite of his professional avocations, and at the evident risk of his precarious health. The numerous facts thus collected are detailed in his Essay on Dew, which appeared in 1814, and immediately attracted a very considerable share of public notice. This little work, however, does not add much to our stock of accurate information, but it is rendered interesting by the variety of collateral objects which it embraces. The experiments themselves rarely display address or delicacy; and Dr Wells, without ever employing the hygrometer or the pyroscope, instruments which he could have then easily procured, generally contents himself with stating merely rude approximations. Fortunately, such coarse results were sufficient to support the main principle, for otherwise they would have required much correction. But we must still regret that the worthy author should have frequently trusted to conjectural reasoning, instead of appealing to direct experiment.
The chief observations collected by Dr Wells may be reduced to a narrow compass. The coldness of the objects exposed was always found to precede the formation of dew, which continued, in favourable circumstances, to accumulate somewhat progressively during the whole night, so that, from midnight to sunrise, the deposition was even greater than from sunset to midnight. Dew was more abundant in the spring and autumn than at other seasons, and it was always very copious when the atmosphere inclined to humidity; for instance, in clear nights succeeding to misty mornings, or in clear mornings succeeding to misty nights.
The coldness which bodies contract from exposure must be augmented by every circumstance which retards the communication of heat. Hence loose and spongy materials are mostly affected. Thus, in a clear night the grass was twelve degrees colder than the garden mould, and sixteen and a half degrees colder than a hard gravel walk. In another bright evening, the surface of snow being nine degrees colder than the air, a piece of swadown laid on it became still four degrees colder. Again, a lock of wool placed on a small table in the garden became nine and a half degrees colder than the air, while swadown, in the same situation, acquired a coldness of eleven and a half degrees.
The quantities of dew which attach to different substances appear to follow the proportions of their relative coldness. Parcels of wool, each weighing ten grains, being teased out into flattened balls of two and a half inches diameter, and laid on a grass plot, on a gravel walk, and on fresh garden mould, acquired, during a clear calm night, respectively, sixteen, nine, and eight grains of humidity. In another favourable night ten grains of wool laid on the table attracted sixteen grains of dew, while another similar parcel, suspended at the same height in the free air, acquired only ten grains; but the former must have also been much colder than the latter, since its confined situation, unlike the open exposition, would check the dissipation of the frigorific impressions. Hence dew is always denser on grass than on the leaves of shrubs.
But the cooling of substances from exposure, though one great source of dew, is not the only cause of its formation. In low fogs, while the ground is scarcely colder than the incumbent bed of air, the humidity yet settles profusely on all bodies, even on the polished surface of metals. From Six's experiments, it appears that, from the height of 200 feet, the temperature of the atmosphere, in fine evenings, decreases regularly about ten degrees, the colder and therefore denser portions being always thrown down to the surface. Hence the reason of the ancient remark, that the surface of dew is more copious in low vales than on the tops of hills, not the But the observations of Dr Wells serve to confirm the general statement. A lock of wool exposed on a table, imbued, during a clear night, sixteen grains of dew; but a similar parcel, placed immediately under the table, and consequently screened from the aspect of the sky, attracted four grains. In the latter case, the mere accumulation of cold air below must have occasioned the aqueous deposition.
It might perhaps have been judged sufficient if Dr Wells had contented himself with assuming the coldness induced on the ground as merely an experimental fact. At any rate, we cannot help regretting that he should have sought the explication of this primary phenomenon from the very loose, cumbrous, and visionary hypothesis of M. Prevost of Geneva, concerning what is gratuitously called radiant heat. We are at a loss indeed to conceive how a speculation so repugnant to all the principles of sound philosophy should at this time have procured any favour, unless it proceeds from the blind admiration which the multitude are prone to entertain for whatever lulls the reasoning faculty, and appears cloudy and mysterious. See Meteorology.