DEW
, a thin light insensible mist, or rain, ascending with a slow motion, and falling while the sun is below the horizon.
To us it appears to differ from rain, as less from more. Its origin and matter are doubtless from the vapours and exhalations that rise from the earth and water. See Exhalation. Some define it a vapour liquesied, and let fall in drops. M. Huet, in one of his letters, shews that dew does not fall, but rises; and others have adopted the same opinion.
M. du Fay made several experiments, first with glasses, then with pieces of cloth stretched horizontally at different heights; and he found that the lower bodies, with their under surfaces, were wetted before those that were placed higher, or their upper surfaces. And Du Fay and Muschenbroek both found, that different substances, and even different colours, receive the dew differently, and some little or not at all.
From the principles laid down under the article EVAPORATION, the several phenomena of dews are easily accounted for. Such as, for instance, that dews are | more copious in the spring, than in the other seasons of the year; there being then a greater stock of vapour in readiness, than at other times, by reason of the small expence of it in the winter's cold and frost. Hence it is too, that Egypt, and some other hot countries, abound with dews throughout all the heats of summer; for the air there being too hot to constipate the vapours in the day-time, they never gather into clouds; and hence they have no rain: but in climates that are excessively hot, the nights are remarkahly cold; so that the vapours raised after sun-set, are readily condensed into dews.
It is natural to conclude, from the different substances which are combined with dew, that it must be either salutary or injurious, both to plants and animals.
It is not easy to ascertain the quantity of dew that rises every night, or in the whole year, because of the winds which disperse it, the rains which carry it down, and other inconveniences: but it is known that it rises in greater abundance after rain than after dry weather, and in warm countries than in cold ones. There are some places in which dew is observed only to ascend, and not to fall; and others again in which it is carried upwards in greater plenty than downwards, being dispersed by the winds.
Dr. Hales made some experiments, to determine the quantity of dew that falls in the night. For this purpose, on the 15th of August, at 7 in the evening, he filled two glazed earthen pans with moist earth; the dimensions of the pans being, 3 inches deep, and 12 inches diameter: and he observes, that the moister the earth, the more dew falls on it in a night; and that more than a double quantity of dew falls on a surface of water, than on an equal surface of moist earth. These pans increased in weight by the night's dew, 180 grains; and decreased in weight by the evaporation of the day, 1 oz 282grs: so that 540 grains more are evaporated from the earth every 24 hours in summer, than the dew that falls in the night; i. e. in 21 days near 26 ounces from a circular area of a foot diameter. Now if 180 grains of dew, falling in one night on such an area, which is equal to 113 square inches, be equally spread on the surface, its depth will be the 159th part of an inch. He likewise found that the depth of dew in a winter's night was the 90th part of an inch. If therefore we allow 159 nights for the extent of the summer's dew, it will in that time amount to one inch in depth; and reckoning the remaining 206 nights for the extent of the winter's dew, it will produce 2.28 inches depth; and the dew of the whole year will amount to 3.28 inches depth. But the quantity which evaporated in a fair summer's day from the same surface, being 1 oz and 282 grs, gives the 40th part of an inch deep for evaporation, which is 4 times as much as fell at night. Dr. Hales observes that the evaporation of a winter's day is nearly the same as in a summer's day; the earth's greater moisture in winter compensating for the sun's greater heat in summer. Hales's Vegetable Statics, vol. 1, pa. 52 of 4th edit. See Evaporation.
Signor Beccaria made several experiments to demonstrate the existence of the electricity that is produced by dew. He observes in general, that such electricity took place in clear and dry weather, during which no strong wind prevailed; and that it depends on the quantity of the dew, as the electricity of the rain depends on the quantity of the rain. He sometimes found that it began before sun-set; at other times not till 11 o'clock at night. Artificial Electricity, Appendix, letter 3.
DE WIT (John), the famous Dutch pensionary, was born at Dort, in 1625; where he prosecuted his studies so diligently, that at 23 years of age, he published Elementa Curvarum Linearum, one of the deepest books in mathematics at that time. After taking his degrees, and travelling, he, in 1650, became pensionary of Dort, and distinguished himself very early in the management of public affairs, which soon after raised him to the rank of pensionary of Holland. After rendering the greatest benesits to his country in many important instances, and serving it in several high capacities, with the greatest ability, diligence, and integrity, by some intrigues of the court, it is said, he and his brother were thrown into prison, from whence they were dragged by the mob, and butchered with the most cruel and savage barbarity.
DIACAUSTIC Curve, or the Caustic by Refraction, is a species of caustic curves, the genesis of which is in the following manner. Imagine an infinite number of rays BA, BM, BD, &c, issuing from the same luminous point B, refracted to or from the perpendicular MC, by the given curve AMD; and so, that CE the sines of the angles of incidence CME, be always to CG the sines of the refracted angles CMG, in a given ratio: then the curve HFN that touches all the refracted rays AH, MF, DN, &c, is called the Diacaustic, or Caustic by Refraction.
DIACOUST ICS, or Diaphonics, the considera tion of the properties of sound refracted in passing through different mediums; that is, out of a denser into a more subtile, or out of a more subtile into a denser medium. See Sound.
