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And on the withered dewlap pour the ale. Id. Who would believe that there were mountaineers Dewlapt like bulls, whose throats had hanging at 'em Wallets of flesh? Id.

That Churchman bears a bounteous mind, indeed; A hand as fruitful as the land that feeds us; His dew falls every where.

Id. Dews and rain are but the returns of moist vapours condensed. Bacon. An bost

Innumerable as the stars of night,

Or stars of morning, dewdrops, which the sun Impearls on every leaf, and every flower. Milton. From the earth a dewy mist

Went up, and watered all the ground, and each Plant of the field.

Id.

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DEW is defined by Dr. Hutton'a thin light insensible mist, or rain, ascending with a slow motion, and falling while the sun is below the horizon.' He adds, that it appears to differ from rain, as less from more' Its origin and matter are doubtless from the vapors and exhalations that rise from the earth and water. See EXHALATION.

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As it appears only during clear nights, when the heavens seem to glow with constellations, the ancients finely imagined it to be actually shed from the stars, and therefore to partake of a pure and celestial essence. Hence,' says Mr. Leslie, ‘the vulgar notion that dew falls, which has prevailed through all ages, and continues to tincture every language.' 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 humifying 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.' Certain abstergent quali-ties were at the same period ascribed to dew. Ammianus Marcellinus says that the health of mountaineers is principally owing to their constant exposure to bracing dews.

It was long disputed whether the dew is formed: from the vapors ascending from the earth during, the night time, or from the descent of such as. have been already raised through the day. M. Huet shows that dew does not fall but rises. Some of the most remarkable experiments in support of this hypothesis are those of Mr. Du Fay of the (Royal) Academy of Sciences at Paris. He supposed, that if the dew ascended, it must wet a body placed low down sooner than one placed on a higher situation; and if a number of bodies were placed in this manner the lowermost would be wetted first, and the rest in like manner, gradually up to the top. To determine this, he placed two ladders against one another, meeting at their tops, spreading wide asunder at the bottom, and so tall as to reach thirty-two feet high. To the several steps of these he fastened large squares of glass like the panes of windows, placing them in such a manner that they should not overshade one another. On the trial it appeared exactly as Mr. Du Fay had apprehended. The lower surface of the first piece of glass was first wetted, then the upper, then the lower surface of the pane next above it; and so on, till all the pieces were wetted to the top. Hence it appeared plain to him, that the dews consisted of the vapors ascending from the earth during the night; which, being condensed by the coldness of the atmosphere, are prevented from being dissipated as in the day-time by the sun's heat. He afterwards tried a similar experiment with pieces of cloth instead of panes of glass, and the result was quite conformable to his expectations. He weighed all the pieces of cloth next morning, to know what quantity of water each had imbibed, and found those that had been placed lowermost considerably heavier than such as had been placed at the top; though he owns that this experiment did not succeed so perfectly as the former. M. Muschenbroeck, wno embraced the contrary opinion, thought he had

invalidated all Mr. Du Fay's proofs, by repeating his experiments with the same success, on a plane covered with sheet lead. But to this M. Du Fay replied, that there was no occasion for supposing the vapor to rise through the lead, nor from that very spot; but that, as it arose from the adjoining open ground, the continual fluctuation of the air could not but spread it abroad, and carry it thither in its ascent. This experiment of M. Muschenbroeck's was not considered sufficient to overthrow those of M. Du Fay. Yet one thing seemed to favor the hypothesis of its descent, i. e. that in cloudy weather there is little or no dew to be observed. And Muschenbroeck, continuing his experiments, made the interesting discovery that dew forms in very different proportions on different bodies, for that it will scarcely adhere to a polished metal surface, while it abounds on glass or porcelain. The color of the substance appeared also, he found, to alter the effects. A piece of red leather acquired, by exposure through the night, twice as much dew as another black or blue piece of the same size. He was afterwards, however, led to attribute this latter circumstance to the coloring matter of the morocco leather used.

M. Du Fay also continued his experiments: and the result was, that on neither side of this controversy was there a sufficient preponderance of proof to decide the question; but the old doctrine of Aristotle on the subject was revived, viz. that dew separates, under certain circumstances, from the air, and becomes attracted to particular bodies; or that the moisture, in which it directly originates, is suspended in the atmosphere by a perfectly chemical process, similar to that by which salts are dissolved in water, heat in both cases being found to increase the solvent power. Professor Leslie's attention was first drawn to the subject as early as the year 1798. By means of his hygrometer he then established the curious fact, that the moisture of air is deposited on glass before it actually reaches the point of saturation. He thus explains, in his valuable Treatise on the Relations of Air to Heat and Moisture, the general result of his investigations at this and a subsequent period:-In fine calm weather, after the rays of the declining sun have ceased to wain 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.' 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.' (pp. 92 and 192).

Some interesting experiments were now made in France, in regard to the tendency M. du Fay had observed in different bodies, to imbibe dew in different proportions. It had long been seen that dew is deposited on glass, when metals in its neighbourhood remain dry; M. Prévost of Montaubon however discovered some new and curious facts relative to this deposition. When thin plates of inetal are fixed on pieces of glass, it sometimes happens that they are as much covered with dew as the glass itself: but more frequently they remain dry; and in this case they are also surrounded by a dry zone. But when the other side of the glass is exposed to dew, the part which is opposite to the metal remains perfectly dry. If the metal be again covered with glass, it will lose its effect in preventing the deposition.

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These experiments may be conveniently confirmed on the glass of a window, when moisture is attaching itself to either of its surfaces. Mr. Prévost remarks that it often happens that dew is deposited externally, even when the air within is warmer than without. A plate of metal fixed internally on the window receives a larger quantity of moisture than the glass, while the space opposite to an external plate remains dry: and, if the humidity is deposited from without, the place opposite the internal plate is also more moistened, while the external plate remains dry and both these circumstances may happen at once with the same result. A small plate fixed externally, opposite to the middle of the internal plate, protects this part of the plate from receiving moisture; and a smaller piece of glass, fixed on the external plate, produces again a central spot of moisture on the internal one: and the same changes may be continued for a number of alternations, until the whole thickness becomes more than half an inch. Gilt paper, with its metallic surface exposed, acts as a metal; but when the paper only is exposed it has no effect. When a plate of metal, on which moisture would have been deposited, is fixed at a small distance from the glass, the moisture is transferred to the surface of the glass immediately under it without affecting the metal: if this plate is varnished on the surface remote from the glass, the effect remains; but if on the side next the glass, it is destroyed. The oxidation of metals renders them also unfit for the experiment. When glasses partly filled with mercury, or even with water, are exposed to the dew, it is deposited only on the parts which are above the surface of the fluid. But in all cases when the humidity is too copious the results are confused. In order to reduce these facts to some general laws, M. Prévost observes, that when the metal is placed on the warmer side of the glass, the humidity is deposited more copiously either on itself or on either surface of the glass in its neighbourhood: but that, when it is on the colder side, it neither receives humidity, nor permits its deposition on

the glass: that a coat of glass, or varnish, destroys the efficacy of the metal, but that an additional plate of metal restores it.

M. Prévost was at first disposed to attribute these phenomena to the effects of electricity, but he thinks it possible to explain them all by the action of heat only; for this purpose he assumes, first, that glass attracts humidity the more powerfully as its temperature is lower; secondly, that metals attract it but very little; thirdly, that glass exerts this attraction, notwithstanding the interposition of other bodies; and, fourthly, that metals give to glass, placed in their neighbourhood, the power of being heated by warm air, and being cooled by cold air, with greater rapidity. Hence, that the temperature of the glass approaches more nearly to that of the air on the side opposite to the metal, and attracts the humidity accordingly, more or less, either to its own surface, or to that of the metal. We should, indeed, have expected a contrary effect; that the metal would rather have tended to communicate to the glass the temperature of the air on its own side; but, granting that the assumptions of M. Prévost serve to generalise the facts with accuracy, their temporary utility is as great as if they were fundamentally probable.

Dr. Wells, however, has traced up the phenomena of dew to their legitimate sources. Very little,' he observes, with Aristotle, 'is deposited, except on calm and clear nights, or when the clouds are high. It is never seen on nights both cloudy and windy; and if, in the course of the night, the weather, from being serene, should become dark and stormy, dew, which had been deposited, will disappear. In calm weather, if the sky be partially covered with clouds, more dew will appear than if it were entirely uncovered.'

Dew probably begins in the country to appear upon grass, in places shaded from the sun, during clear and calm weather, soon after the heat of the atmosphere has declined, and continues to be deposited through the whole night, and for a little after sun-rise. Its quantity will depend, in some measure, on the proportion of moisture in the atmosphere, and is, consequently, greater after rain than after a long tract of dry weather; and in Europe, with southerly and westerly winds, than with those which blow from the north and the east. The direction of the sea determines this relation of the winds to dew. For in Egypt, dew is scarcely ever observed, except while the northerly or Etesian winds prevail. Hence, also, dew is generally more abundant in spring and autumn, than in summer. And it is always very copious on those clear nights which are followed by misty mornings, which show the air to be loaded with moisture. And a clear morning, following a cloudy night, determines a plentiful deposition of the retained vapor. When warmth of atmosphere is compatible with clearness, as is the case in southern latitudes, though seldom in our country, the dew becomes much more copious, because the air then contains more moisture. Dew continues to form with great copiousness, as the night advances, from the increased refrigeration of the ground. Dew, according to Aristotle, is 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. Opinions of this kind, says Dr. Wells, are still entertained by many persons, among whom is the very ingenious professor, Leslie. (Relations of Heat and Moisture, pp. 37 and 132). A fact, however, first taken notice of by Gerstin, who published his Treatise on Dew in 1773, proves them to be erroneous; for he found that bodies a little elevated in the air, often become moist with dew, while similar bodies, lying on the ground, remain dry, though necessarily, from their position, as liable to be wetted, by whatever falls from the heavens, as the former. The above notion is perfectly refuted by what will presently appear relative to metallic surfaces exposed to the air in a horizontal position, which remain dry, while every thing around them is covered with dew.

After a long period of drought, when the air was very still and the sky serene, Dr. Wells exposed to the sky, twenty-eight minutes before sun-set, previously weighed parcels of wool and swandown, upon a smooth, unpainted, and perfectly dry fir-table, five feet long, three broad, and nearly three in height, which had been placed, an hour before, in the sunshine, in a large level grass-field. The wool, twelve minutes after sun-set, was found to be 14° colder than the air, and to have acquired no weight. The swandown, the quantity of which was much greater than that of the wool, was, at the same time, 13° colder than the air, and was also without any additional weight. In twenty minutes more, the swandown was 14° 30′ colder than the neighbouring air, and was still without any increase of its weight. At the same time the grass was 15° colder than the air four feet above the ground.

Dr. Wells, by a copious induction of facts, derived from observation and experiment, establishes the proposition, that bodies become colder than the neighbouring air before they are dewed. The cold, therefore, which Dr. Wilson and Mr. Six conjectured to be the effect of dew, now appears to be its cause. But what makes the terrestrial surface colder than the atmosphere? The radiation or projection of heat into free space. Now the researches of professor Leslie and count Rumford have demonstrated, that different bodies project heat with very different degrees of force.

In the operation of this principle, therefore, conjoined with the power of a concave mirror of cloud, or any other awning, to reflect, or throw down again those calorific emanations which would be dissipated in a clear sky, we shall find a solution of the most mysterious phenomena of dew. Two circumstances must here be considered:

I. The exposure of the particular surface to be dewed, to the free aspect of the sky.

II. The peculiar radiating power of the surface. 1. Whatever diminishes the view of the sky, as seen from the exposed body, obstructs the depression of its temperature, and occasions the quantity of dew formed upon it, to be less than would have occurred, if the exposure to the sky had been complete.

Dr. Wells bent a sheet of pasteboard into the the tin-foil prevents the glass under it from dis shape of a pent-house, making the angle of sipating its heat, and, therefore, it can receive flexure 90°, and leaving both ends open. This no dew; in the second case, the tin-foil prevents was placed one evening, with its ridge upper- the glass, which it coats, from receiving the most, upon a grass-plat, in the direction of the calorific influence of the apartment, and hence it wind, as well as this could be ascertained. He is sooner refrigerated by external radiation than then laid ten grains of white, and moderately the rest of the pane. Gold, silver, copper, and fine wool, not artificially dried, on the middle tin, bad radiators of heat and excellent conpart of that spot of the grass which was sheltered ductors, acquire dew with greater difficulty by the roof, and the same quantity on another than platina, which is a more imperfect conpart of the grass-plat, fully exposed to the sky. ductor; or than lead, zinc, and steel, which are In the morning, the sheltered wool was found to better radiators. Hence, dew which has formed have increased in weight only two grains, but upon a metal will often disappear, while other that which had been exposed to the sky, sixteen substances in the neighbourhood remain wet; grains. He varied the experiment on the same and a metal, purposely moistened, will become night, by placing, upright, on the grass-plat, a dry, while neighbouring bodies are acquiring hollow cylinder of baked clay, one foot diameter, moisture. This repulsion of dew is communiand two feet and a-half high. On the grass cated by metals to bodies in contact with or near round the outer edge of the cylinder, were laid them. Wool laid on metal acquires less dew ten grains of wool, which, in this situation, as than wool laid on the contiguous grass. there was not the least wind, would have received as much rain as a like quantity of wool fully exposed to the sky. But the quantity of moisture acquired by the wool partially screened by the cylinder from the aspect of the sky, was only about two grains, while that acquired by the same quantity, fully exposed, was sixteen grains. Repose of a body seems necessary to its acquiring its utmost coolness, and a full deposit of dew. Gravel-walks and pavements project heat, and acquire dew, less readily than a grassy surface. Hence, wool placed on the former, has its temperature less depressed than on the latter, and, therefore, is less bedewed. Nor does the wool here attract moisture by capillary action on the grass, for the same effect happens if it be placed in a saucer. Nor is it by hydrometric attraction; for, in a cloudy night, wool placed on an elevated board acquired scarcely any increase of weight.

If wool be insulated a few feet from the ground, on a bad conductor of heat, as a board, it will become still colder than when in contact with the earth, and acquire fully more dew than on the grass. At the windward end of the board it is less bedewed than at the sheltered end, because, in the former case, its temperature is nearer to that of the atmosphere. Rough and porous surfaces, as shavings of wood, take more dew than smooth and solid wood; and raw silk and fine cotton are more powerful in this respect than even wool. Glass projects heat rapidly, and is as rapidly coated with dew. But bright metals attract dew much less powerfully than other bodies. If we coat a piece of glass, partially, with bright tin-foil, or silver leaf, the uncovered portion of the glass quickly becomes cold by radiation, on exposure to a clear nocturnal sky, and acquires moisture; which, beginning on those parts most remote from the metal, gradually approaches it. Thus, also, if we coat outwardly a portion of a window-pane with tin-foil, in a clear night, then moisture will be deposited inside, on every part except opposite to the metal. But if the metal be inside, then the glass under and beyond it will be sooner, or most copiously bedewed. In the first case,

If the night becomes cloudy, after having been very clear, though there be no change with respect to calmness, a considerable alteration in the temperature of the grass always ensues. Upon one such night, the grass, after having been 12° colder than the air, became only 2° colder; the atmospheric temperature being the same at both observations. On a second night, the grass became 9° warmer in the space of an hour and a half. On a third night, in less than forty-five minutes, the temperature of the grass rose 15°, while that of the neighbouring air increased only 34°. During a fourth night, the temperature of the grass, at half past nine o'clock, was 32°. In twenty minutes afterwards, it was found to be 39°, the sky in the mean time having become cloudy. At the end of twenty minutes more, the sky being clear, the temperature of the grass was again 32°. A thermometer lying on a grass-plat will sometimes rise several degrees, when a cloud comes to occupy the zenith of a clear sky.

When, during a clear and still night, different thermometers, placed in different situations, were examined at the same time, those which were situated where most dew was formed, were always found to be the lowest. On dewy nights the temperature of the earth, half an inch or an inch beneath the surface, is always found much warmer than the grass upon it, or the air above it. The differences on hve such nights, were from 12° to 16°.

In making experiments with thermometers, it is necessary to coat their bulbs with silver or gold leaf, otherwise the glassy surface indicates a lower temperature than that of the air, or the metallic plate it touches. Swandown seems to exhibit greater cold, on exposure to the aspect of a clear sky, than any thing else. When grass is 14° below the atmospheric temperature, swandown is commonly 15°. Fresh unbroken straw and shreds of paper, rank in this respect with swandown. Charcoal, lamp-black, and rust of iron, are also very productive of cold. Snow stands 4° or 5° higher than swandown laid upon it in a clear night.

The following tabular view of observations by Dr. Wells, is peculiarly instructive :

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Heat of the air four feet above the grass,

wool on a raised board,
swandown on the same,
surface of the raised board,
grass-plat,

The temperature always falls in clear nights; but the deposition of dew, depending on the moisture of the air, may occur or not. Now, if cold were the effect of dew, the cold connected with dew ought to be always proportional to the quantity of that fluid; but this is contradicted by experience. On the other hand, if it be granted that dew is water precipitated from the atmosphere by the cold of the body on which it appears, the same degree of cold in the precipitating body may be attended with much, with little, or with no dew, according to the existing state of the air in regard to moisture; all of which circumstances are found really to take place. The actual precipitation of dew, indeed, ought to evolve heat.

A very few degrees of difference of temperature between the grass and the atmosphere are sufficient to determine the formation of dew, when the air is in a proper state. But a difference of even 30°, or more, sometimes exists, by the radiation of heat from the earth to the heavens. And hence, the air near the refrigerated surface must be colder than that somewhat elevated. Agreeably to Mr. Six's observations, the atmosphere, at the height of 220 feet, is often, upon such nights, 10° warmer than what it is seven feet above the ground. And had not the lower air thus imparted some of its heat to the surface, the latter would have been probably 40° under the temperature of the air.

Insulated bodies, or prominent points, are sooner covered with hoar-frost and dew than others; because the equilibrium of their temperature is more difficult to be restored. As aerial stillness is necessary to the cooling effect of radiation, we can understand why the hurtful effects of cold, heavy fogs, and dews, occur chiefly in hollow and confined places, and less frequently on hills. In like manner, the leaves of trees often remain dry throughout the night, while the blades of grass are covered with dew.

No direct experiments can be made to ascertain the manner in which clouds prevent or lessen the appearance of a cold at night, upon the surface of the earth, greater than that of the atmosphere. But it may be concluded from the preceding observations, that they produce this effect almost entirely by radiating heat to the earth, in return for that which they intercept in its progress from the earth towards the heavens. The heat extricated by the condensation of transparent vapor into cloud must soon be dissipated; whereas, the effect of greatly lessening, or preventing altogether, the appearance of a greater cold on the earth than that of the air, will be produced by a cloudy sky during the whole of a long night.

We can thus explain, in a more satisfactory manner than has usually been done, the sudden

warmth that is felt in winter, when a fleece of clouds supervenes in clear frosty weather. Chemists ascribed this sudden and powerful change to the disengagement of the latent heat of the condensed vapors; but Dr. Wells's thermometric observations on the sudden alternations of temperature by cloud and clearness, render that opinion untenable. We find the atmosphere itself, indeed, at moderate elevations, of pretty uniform temperature, while bodies at the surface of the ground suffer great variations in their temperature. This single fact is fatal to the hypothesis derived from the doctrines of latent heat.

'I had often,' says Dr. Wells, 'smiled, in the pride of half knowledge, at the means frequently employed by gardeners to protect tender plants from cold, as it appeared to me impossible that a thin mat, or any such flimsy substance, could prevent them from attaining the temperature of the atmosphere, by which alone I thought them liable to be injured. But when I had learned that bodies on the surface of the earth become, during a still and serene night, colder than the atmosphere, by radiating their heat to the heavens, I perceived immediately a just reason for the practice which I had before deemed useless. Being desirous, however, of acquiring some precise information on this subject, I fixed perpendicularly, in the earth of a grass-plat, four small sticks, and over their upper extremities, which were six inches above the grass, and formed the corners of a square whose sides were two feet long, I drew tightly a very thin cambric handkerchief. In this disposition of things, therefore, nothing existed to prevent the free passage of air from the exposed grass to that which was sheltered, except the four small sticks, and there was no substance to radiate downwards to the latter grass, except the cambric handkerchief.'

The sheltered grass, however, was found nearly of the same temperature as the air, while the unsheltered was 5° or more colder. One night the fully exposed grass was 11° colder than the air; but the sheltered grass was only 3° colder. Hence we see the power of a very slight awning to avert or lessen the injurious coldness of the ground. To have the full advantage of such protection from the chill aspect of the sky, the covering should not touch the subjacent bodies. Garden walls act partly on the same principle. Snow screens plants from this chilling radiation. In warm climates, the deposition of dewy moisture on animal substances hastens their putrefaction. As this is apt to happen only in clear nights, it was anciently supposed that bright moonshine favored animal corruption.

From this rapid emission of heat from the surface of the ground, we can now explain the formation of ice during the night in Bengal, while the temperature of the air is above 32o. The

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