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Feed him with apricocks and dewberries,
Dew is defined by Dr. Hutton'a thin light insenWith purple grapes, green figs, and mulberries. sible mist, or rain, ascending with a slow motion,
and falling while the sun is below the horizon.' I must go seek some dewdrops here,
He adds, that it appears to differ from rain, as And hang a pearl in every cowslip's ear. Id.
less from more? Its origin and matter are doubtAnd sometimes lurk I in a gossip's bowl,
less from the vapors and exhalations that rise In very likeness of a roasted crab;
from the earth and water. See ExHaLATION. And when she drinks against her lips I bob, And on the withered dewlap pour the ale.
As it appears only during clear nights, when
the heavens seem to glow with constellations, Who would believe that there were mountaineers
the ancients finely imagined it to be actually shed Deurlapt like balls, whose throats had hanging at 'em
from the stars, and therefore to partake of a Wallets of besh ?
pure and celestial essence. “Hence,' says Mr. That Churchman bears a bounteous mind, indeed; Leslie, the vulgar notion that dew falls, which A hand as fruitful as the land that feeds us; His dew falls every where.
has prevailed through all ages, and continues to Id.
tincture every language.' Plutarch asserts it to Deur and rain are but the returns of moist vapours be most abundant in the time of full moon. The condensed.
lunar beams themselves were supposed to contriAn bost
bute some influence, being of a cold nature, Innumerable as the stars of night,
and therefore possessed of a humifying quaOr stars of morning, dewdrops, which the sun
lity. The moon, it was imagined, performed Impearls on every leaf, and every flower. Milton.
merely the office of an imperfect mirror, reflectFrom the earth a dewy mist
ing the softened lustre of the sun without any, Went up, and watered all the ground, and each Plant of the field.
portion of his heat.' Certain abstergent quali-
ties were at the same period ascribed to dew. He ceased; discerning Adam with such joy
Ammianus Marcellinus says that the health of Surcharged, as had, like grief, been dewed in tears,
mountaineers is principally owing to their conWithout the vent of words, which these he breathed.
stant exposure to bracing dews. This evening late, by then the chewing flocks It was long disputed whether the dew is formed Had ta’en their supper on the savoury herb
from the vapors ascending from the earth during Of knot-grass dewbesprent, and were in fold, the night time, or from the descent of such as I sat me down to watch upon a bank
have been already raised through the day. M. With ivy canopied, and interwove
Huet shows that dew does not fall but rises. With faunting honey-suckle.
Some of the most remarkable experiments in Dewberries, as they stand here among the more de- support of this hypothesis are those of Mr. Du licate fruits, must be understood to mean raspberries, Fay of the (Royal) Academy of Sciences at which are also of the bramble kind. Hanmer. Paris. He supposed, that if the dew ascended,
For the trout, the dew-worm, which some call the it must wet a body placed low down sooner than lob worm, and the brandling, are the chief. Walton. one placed on a higher situation; and if a numa Palemon above the rest appe: rs
ber of bodies were placed in this manner the In sable garments, dewed with gushing tears. lowermost would be wetted first, and the rest
Dryden. in like manner, gradually up to the top. To Where two adverse winds,
determine this, he placed two ladders against Sallimed from dewy vapours in mid sky, one another, meeting at their tops, spreading Engage with horrid shock, the ruffled brine
wide asunder at the bottom, and so tall as to Roars stormy.
reach thirty-two feet high. To the several steps In Gallic blood again
of these he fastened large squares of glass like the He deus his reeking sword, and strows the ground
panes of windows, placing them in such a manWith beardless ranks.
ner that they should not overshade one another. Large rowles of fat about his shoulder slung,
On the trial it appeared exactly as Mr. Du Fay And from his neck the double dewlap hung.
had apprehended. The lower surface of the first Addison.
piece of glass was first wetted, then the upper, The dewlapt bull now chases along the plain,
then the lower surface of the pane next above it; While burning love ferments in every vein. Gay.
and so on, till all the pieces were wetted to the Rest, sweet as dewdrops on the flowery lawns, When the sky opens, and the morning dawns.
top. Hence it appeared plain to him, that the Tickell.
dews consisted of the vapors ascending from the Now sliding streams the thirsty plants renew,
earth during the night; which, being condensed
by the coldness of the atmosphere, are prevented And feed their fibres with reviviny dew. Pope.
from being dissipated as in the day-time by the No more the morn, with tepid rays,
sun's heat. He afterwards tried a similar expeI'nfolds the flower of various hue,
riment with pieces of cloth instead of panes of Noon spreads no more the genial blaze, Nor gentle eve distils the dew.
glass, and the result was quite conformable to Johnson, Ode to Wintcr.
his expectations. He weighed all the .pieces of The spring is come; the violet's gone,
cloth next morning, to know what quantity of The firsd-born child of the early sun;
water each bad imbibed, and found those thar With us she is but a winter's flower,
had been placed lowermost considerably heavier The snow on the hills cannot blast her bower,
than such as had been placed at the top; though And she lifts up her deroy eye of blue
he owns that this experiment did not succeed so To, the youngest sky of the self-same huc. perfectly as the former. M. Muschenbroeck, wno
Byron. embraced the contrary opinion, thought he had
invalidated all Mr. Du Fay's proofs, by repeating clouded, less alteration is betrayed in the state of his experiments with the same success, on a plane the air, both during the progress of the day, and covered with sheet lead. But to this M. Du at different distances from the ground; and, if Fay replied, that there was no occasion for sup- wind prevail, the lower strata of the atmosphere, posing the vapor to rise through the lead, nor thus agitated and intermingled, will be reduced from that very spot; but that, as it arose from ' to a still nearer equality of condition.' (pp. 92 the adjoining open ground, the continual fluc- and 192). tuation of the air could not but spread it abroad, Some interesting experiments were now made and carry it thither in its ascent. This experi- in France, in regard to the tendency M. du Fay ment of M. Muschenbroeck's was not considered had observed in different hodies, to imbibe dew sufficient to overthrow those of M. Du Fay. Yet in different proportions. It had long been seen one thing seemed to favor the hypothesis of its that dew is deposited on glass, when metals in descent, i. e. that in cloudy weather there is its neighbourhood remain dry ; M. Prévost of little or no dew to be observed. And Muschen- Montaubon bowever discovered some new and broeck, continuing his experiments, made the curious facts relative to this deposition. When interesting discovery that dew forms in very dif- thin plates of inetal are fixed on pieces of glass, ferent proportions on different bodies, for that it it sometimes happens that they are as much will scarcely adhere to a polished metal surface, covered with dew as the glass itself : but more while it abounds on glass or porcelain. The frequently they remain dry; and in this case they color of the substance appeared also, he found, are also surrounded by a dry zone. But when to alter the effects. A piece of red leather ac- the other side of the glass is exposed to dew, the quired, by exposure through the night, twice as part which is opposite to the metal remains permuch dew as another black or blue piece of the fectly dry. If the metal be again covered with same size. He was afterwards, however, led to glass, it will lose its effect in preventing the deattribute this latter circumstance to the coloring position. matter of the morocco leather used.
These experiments may be conveniently conM. Du Fay also continued his experiments: firmed on the glass of a window, when moisture and the result was, that on neither side of this is attaching itself to either of its surfaces. Mr. controversy was there a sufficient preponderance Prévost remarks that it often happens that dew is of proof to decide the question ; but the old doc- deposited externally, even when the air within is trine of Aristotle on the subject was revived, viz. 'warmer than without. A plate of metal fixed inthat dew separates, under certain circumstances, ternally on the window receives a larger quantity from the air, and becomes attracted to particular of moisture than the glass, while the space oppobodies; or that the moisture, in which it directly site to an external plate remains dry : and, if the eriginates, is suspended in the atmosphere by a humidity is deposited from without, the place opperfectly chemical process, similar to that by posite the internal plate is also more moistened, which salts are dissolved in water, heat in both while the external plate remains dry: and both cases being found to increase the solvent power. these circumstances may happen at once with the
Professor Leslie's attention was first drawn to same result. A small plate fixed externally, the subject as early as the year 1798. By means opposite to the middle of the internal plate, proof his hygrometer he then established the curious tects this part of the plate from receiving moisfact, that the moisture of air is deposited on glass ture; and a smaller piece of glass, fixed on the before it actually reaches the point of saturation. external plate, produces again a central spot of He thus explains, in his valuable Treatise on the moisture on the internal one: and the same Relations of Air to Heat and Moisture, the gene- changes may be continued for a number of alterral result of his investigations at this and a sub- nations, until the whole thickness becomes more sequent period :- In fine calm weather, after than half an inch. Gilt paper, with its metallic the rays of the declining sun have ceased to surface exposed, acts as a metal; but when the wann the surface of the ground, the descent of paper only is exposed it has no effect. When the higher mass of air gradually chills the under- a plate of metal, on which moisture would have most stratum, and disposes it to dampness, till been deposited, is fixed at a small distance from their continued intermixture produces a fog, or the glass, the moisture is transferred to the surlow cloud. Such fogs are, towards the evening, face of the glass immediately under it without often observed gathering in narrow vales, or along affecting the metal : if this plate is varnished on the course of sluggish rivers, and generally hover- the surface remote from the glass, the effect reing within a few inches of the surface. But in mains; but if on the side next the glass, it is all situations, these watery deposits, either to a destroyed. The oxidation of metals renders them greater or a less degree, occur in the same dispo- also unfit for the experiment. When glasses sition of the atmosphere. The minute suspended partly filled with mercury, or even with water, globules, attaching themselves to the projecting are exposed to the dew, it is deposited only on points of the herbage, form dew in mild weather, the parts which are above the surface of the fluid. or shoot into hoar-frost when cold predominates. But in all cases when the humidity is too copious They collect most readily on glass, but seem to the results are confused. In order to reduce be repelled by a bright surface of metal.' In these facts to some general laws, M. Prévost obclear and calm weather, the air is always drier serves, that when the metal is placed on the near the surface during the day than at a certain warmer side of the glass, the humidity is deheight above the ground, but it becomes damper posited more copiously either on itself or on on the approach of evening, while, at some eleva- either surface of ihe glass in its neighbourhood : tion, it retains a moderate degree of dryness but that, when it is on the colder side, it neither through the whole of the night. If the sky be receives humidity, nor permits its deposition on
the giass: that a coat of glass, or varnish, destroys rain, formed in the lower atmosphere, in consethe efficacy of the metal, but that an additional quence of its moisture being condensed, by the plate of metal restores it.
cold of the night, into minute drops. Opinions M. Prévost was at first disposed to attribute of this kind, says Dr. Wells, are still entertained these phenomena to the effects of electricity, but by many persons, among whom is the very ingeLe thinks it possible to explain them all by the nious professor, Leslie. (Relations of Heat and action of beat only; for this purpose he assumes, Moisture, pp. 37 and 132). A fact, however, first, that glass attracts humidity the more power- first taken notice of by Gerstin, who published fully as its temperature is lower; secondly, that his Treatise on Iew in 1773, proves them to be meials attract it but very little; thirdly, that erroneous; for he found that bodies a little glass exerts this attraction, notwithstanding the elevated in the air, often become moist with dew, interposition of other bodies; and, fourthly, that while similar bodies, lying on the ground, remetals give to glass, placed in their neighbour- main dry, though necessarily, from their position, hood, the power of being heated by warm air, as liable to he wetted, by whatever falls from the and being cooled by cold air, with greater rapi- heavens, as the former. The above notion is dity. Hence, that the temperature of the glass perfectly refuted by what will presently appear approaches more nearly to that of the air on the relative to metallic surfaces exposed to the air in side opposite to the metal, and attracts the humi- a horizoutal position, which remain dry, while dity accordingly, more or less, either to its own every thing around them is covered with dew. surface, or to that of the metal. We should, After a long period of drought, when the air indeed, have expected a contrary effect; that was very still and the sky serene, Dr. Wells exthe metal would rather have tended to communi- posed to the sky, twenty-eight minutes before cate to the glass the temperatare of the air on its sun-set, previously weighed parcels of wool and own side; but, granting that the assumptions of swandown, upon a sinooth, unpainted, and M. Prévost serve to generalise the facts with perfectly dry fir-table, five feet long, three accuracy, their temporary utility is as great as if broad, and nearly three in height, which had been they were fundamentally probable.
placed, an hour before, in the sunshine, in a Dr. Wells, however, has traced up the pheno- large level grass-field. The wool, twelve miElena of dew to their legitimate sources. Very nutes after sun-set, was found to be 14° colder little,' he observes, with Aristotle, 'is deposited, than the air, and to have acquired no weight. except on calm and clear nights, or when the The swandown, the quantity of which was much clouds are high. It is never seen on nights both greater than that of the wool, was, at the same
loudy and windy; and if, in the course of the time, 13° colder than the air, and was also night, the weather, from being serene, should be- without any additional weight. In twenty micome dark and stormy, dew, which had been nutes more, the swandown was 14° 30' colder deposited, will disappear. In calm weather, if than tne neighbouring air, and was still without the sky be partially covered with clouds, more any increase of its weight. At the same time the dew will appear than if it were entirely un- grass was 150 colder than the air four feet above. covered.'
the ground. Dew probably begins in the country to appear Dr. Wells, by a copious induction of facts, upon grass, in places shaded from the sun, derived froin observation and experiment, estaduring clear and calm weather, soon after the blishes the proposition, that bodies become beat of the atmosphere has declined, and conti- colder than the neighbouring air before they are nues to be deposited through the whole night, dewed. The cold, therefore, which Dr. Wilson, and for a little after sun-rise. Its quantity will and Mr. Six conjectured to be the effect of dew, depend, in some measure, on the proportion of now appears to be its cause. But what makes moisture in the atmosphere, and is, conse- the terrestrial surface colder than the atmosphere? quently, greater after rain than after a long tract The radiation or projection of heat into free of dry weather; and in Europe, with southerly space. Now the researches of professor Leslie and westerly winds, than with those which blow and count Rumford have demonstrated, that diffrom the north and the east. The direction of ferent bodies project heat with very different dethe sea determines this relation of the winds to grees of force. dew. For in Egypt, dew is scarcely ever ob In the operation of this principle, therefore, served, except while the northerly or Etesian winds conjoined with the power of a concave mirror of prevail
. Hence, also, dew is generally more cloud, or any other awning, to reflect, or throw abundant in spring and autumn, than in summer. down again those calorific emanations which And it is always very copious on those clear would be dissipated in a clear sky, we shall find nights which are followed by misty mornings, a solution of the most mysterious phenomena of which show the air to be loaded with moisture. dew. Two circumstances must here be consiAnd a clear morning, following a cloudy night, dered :determines a plentiful deposition of the retained I. The exposure of the particular surface to vapor. When warmth of atmosphere is com- be dewed, to the free aspect of the sky, patible with clearness, as is the case in southern II. The peculiar radiating power of the surlatitudes, though seldom in our country, the dew face. 1. Whatever diminishes the view of the becomes much more copious, because the air sky, as seen from the exposed body, obstructs then contains more moisture. Dew continues to the depression of its temperature, and occasions form with great copiousness, as the night advan- the quantity of dew formed upon it, to be less ces, from the increased refrigeration of the ground. than would have occurred, if the exposure to the
Dew, according to Aristotle, is a species of 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 fexure 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.płat, in the direction of the caloritic 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, vot 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. O.n 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 re If the night becomes cloudy, after having been ceived as much rain as a like quantity of wool very clear, though there be no change with refully exposed to the sky. But the quantity of spect to calmness, a considerable alteration in moisture acquired by the wool partially screened the temperature of the grass always ensues. by the cylinder from the aspect of the sky, was Upon one such night, the grass, after having been only about two grains, while that acquired by 12° colder than the air, became only 2° colder; the the same quantity, fully exposed, was sixteen atmospheric temperature being the same at both grains. Repose of a body seems necessary to its observations. On a second night, the grass beacquiring its utmost coolness, and a full deposit came 9° warmer in the space of an hour and a of dew. Gravel-walks and pavements project half. On a third night, in less than forty-five heat, and acquire dew, less readily than a grassy minutes, the temperature of the grass rose 15°, surface.
Hence, wool placed on the former, has while that of the neighbouring air increased only its temperature less depressed than on the latter, 31°. During a fourth night, the temperature of and, therefore, is less bedewed. Nor does the the grass, at half past nine o'clock, was 32°. Io wool here attract moisture by capillary action on twenty minutes afterwards, it was found to be the grass, for the same effect happens if it be 39°, the sky in the mean time having become placed in a saucer. Nor is it by hydrometric cloudy. At the end of twenty minutes more, the attraction; for, in a cloudy night, wool placed sky being clear, the temperature of the grass was on an elevated board acquired scarcely any in- again 32o. A thermometer lying on a grass-plat crease of weight.
will sometimes rise several degrees, when a cloud If wool be insulated a few feet from the comes to occupy the zenith of a clear sky. ground, on a bad conductor of heat, as a board, When, during a clear and still night, different it will become still colder than when in contact thermometers, placed in different situations, were with the earth, and acquire fully more dew than examined at the same time, those which were on the grass. At the windward end of the situated where most dew was formed, were always board it is less bedewed than at the sheltered found to be the lowest. On dewy nights the erid, because, in the former case, its temperature temperature of the earth, half an inch or an inch is nearer to that of the atmosphere. Rough and beneath the surface, is always found much porous surfaces, as shavings of wood, take more warmer than the grass upon it, or the air above dew than smooth and solid wood; and raw silk it. The differences on hve such nights, were and fine cotton are more powerful in this respect from 12° to 16o. than even wool. Glass projects heat rapidly, In making experiments with thermometers, it is and is as rapidly coated with dew. But bright necessary to coat their bulbs with silver or gold metals attract dew much less powerfully than leaf, otherwise the glassy surface indicates a lower other bodies. If we coat a piece of glass, par- temperature than that of the air, or the metallic tially, with bright tin-foil, or silver leaf, the un- plate it touches. Swandown seems to exhibit covered portion of the glass quickly becomes greater cold, on exposure to the aspect of a clear cold by radiation, on exposure to a clear noc- sky, than any thing else. When grass is 14o beturnal sky, and acquires moisture; which, be low the atmospheric temperature, swandown is ginning on those parts most remote from the commonly 15o. Fresh unbroken straw and metal, gradually approaches it. Thus, also, if shreds of paper, rank in this respect with swanwe coat outwardly a portion of a window-pane down. Charcoal, lamp-black, and rust of iron, with tin-foil, in a clear night, then moisture will are also very productive of cold. Snow stands be deposited inside, on every part except oppo- 4° or 5° higher than swandown laid upon it in a site to the metal. But if the metal be inside, clear night. then the glass under and beyond it will be sooner, The following tabular view of observations by or most copiously bedewed. In the first case, Dr. Wells, is peculiarly instructive :
The temperature always falls in clear nights; warmth that is felt in winter, when a fleece of but the deposition of dew, depending on the clouds supervenes in clear frosty weather. Chemoisture of the air, may occur or not. Now, if mists ascribed this sudden and powerful change cold were the effect of dew, the cold connected to the disengagement of the latent heat of the with dew ought to be always proportional to the condensed vapors; but Dr. Wells's thermometric quantity of that fuid ; but this is contradicted observations on the sudden alternations of temby experience. On the other hand, if it be perature by cloud and clearness, render that granted that dew is water precipitated from the opinion untenable. We find the atmosphere atmosphere by the cold of the body on which it itself, indeed, at moderate elevations, of pretty appears, the same degree of cold in the precipi- uniform temperature, while bodies at the surface tating body may be attended with much, with of the ground suffer great variations in their temlittle, or with no dew, according to the existing perature. This single fact is fatal to the hypostate of the air in regard to moisture; all of thesis derived from the doctrines of latent heat. which circumstances are found really to take 'I had often,' says Dr. Wells, ‘smiled, in the place. The actual precipitation of dew, indeed, pride of half knowledge, at the means frequently ought to evolve heat.
employed by gardeners to protect tender plants A very few degrees of difference of tempera- from cold, as it appeared to me impossible that a ture between the grass and the atmosphere are thin mat, or any such flimsy substance, could sufficient to determine the formation of dew, prevent them from attaining the temperature of when the air is in a proper state. But a differ- the atmosphere, by which alone I thought them ence of even 30°, or more, sometimes exists, by liable to be injured. But when I had learned the radiation of heat from the earth to the that bodies on the surface of the earth become, heavens. And hence, the air near the refrige- during a still and serene night, colder than the rated surface must be colder than that somewhat atmosphere, by radiating their heat to the elevated. Agreeably to Mr. Six's observations, heavens, I perceived immediately a just reason the atmosphere, at the height of 220 feet, is often, for the practice which I had before deemed useupon such nights, 10° warmer than what it is less. Being desirous, however, of acquiring seren feet above the ground. And had not the some precise information on this subject, I fixed lower air thus imparted some of its heat to the perpendicularly, in the earth of a grass-plat, four surface, the latter would have been probably 40° small sticks, and over their upper extremities, under the temperature of the air.
which were six inches above the grass, and Insulated bodies, or prominent points, are formed the corners of a square whose sides were sooner covered with hoar-frost and dew than two feet long, I drew tightly a very thin cambric others; because the equilibrium of their tempe- handkerchief. In this disposition of things, thererature is more difficult to be restored. As aerial fore, nothing existed to prevent the free passage stillness is necessary to the cooling effect of ra- of air from the exposed grass to that which was diation, we can understand why the hurtful ef- sheltered, except the four small sticks, and there fects of cold, heavy fogs, and dews, occur chiefly was no substance to radiate downwards to the in hollow and confined places, and less frequently latter grass, except the cambric handkerchief.' on hills. In like manner, the leaves of trees The sheltered grass, however, was found nearly often remain dry throughout the night, while the of the same temperature as the air, while the unblades of grass are covered with dew.
sheltered was 50 or more colder. One night the No direct experiments can be made to ascer- fully exposed grass was 11° colder than the air; tain the manner in which clouds prevent or lessen but the sheltered grass was only 30 colder. the appearance of a cold at night, upon the sur- Hence we see the power of a very slight awning face of the earth, greater than that of the atmo to avert or lessen the injurious coldness of the sphere. But it may be concluded from the pre- ground. To have the full advantage of such ceding observations, that they produce this effect protection from the chill aspect of the sky, the almost entirely by radiating heat to the earth, in covering should not touch the subjacent bodies. return for that which they intercept in its pro- Garden walls act partly on the same principle. gress from the earth towards the heavens. The Snow screens plants from this chilling radiation. heat extricated by the condensation of transpa- In warm climates, the deposition of dewy moisrent rapor into cloud must soon be dissipated; ture on animal substances hastens their putrefacwhereas, the effect of greatly lessening, or pre- tion. As this is apt to happen only in clear venting altogether, the appearance of a greater nights, it was anciently supposed that bright cold on the earth than that of the air, will be moonshine favored animal corruption. produced by a cloudy sky during the whole of a From this rapid emission of heat from the long night.
surface of the ground, we can now explain the We can thus explain, in a more satisfactory formation of ice during the night in Bengal, while manner than has usually been done, the sudden the temperature of the air is above 320. The