DEW (AS. thatc, Ger. Than ; ef. Skt. dime, (Mar, to flow). Dew is said to be formed when atmospheric moisture is condensed by cold into drops of water upon grass, trees. rocks, roofs of buildings. or other solid surfaces, while the adjacent air remains clear. The moisture which gathers on the outside of a vessel containing cold water is also termed dew. A thermometer placed in the cold water within this vessel so as to give the temperature of the external sur face on which the dew is deposited. indicates the so-called temperature of the dew-point. or the temperature to which the free air must be cooled in order to begin depositing dew. and this combination of thermometer and vessel is a dew-point apparatus. nmy forms of such apparatus have been devised in order to obtain the highest accuracy in the determination of this important meteorological datum and are de scribed in Abbe's Meteoro/ogica/ Apparatus and 18ss). The apparatus usually preferred is that of Regnault. which, with occasional slight modifications, is used in all determinations of atuotspiterie moisture. This apparatus•consist, essentially of a very sensitive thermometer, whose bulb dips into a thimble of polished gold or silver. while the upper end of the graduated stem protrudes beyond this thimble. if the thimble is tilled with ether and a eurrent of air blown through it in order to evaporate the ether. the latter soon eonls down to the dew-point. Mien this is reached a slight deposit of dew is, seen on the outside of the polished thimble. At this moment the reading of the thernmmete• is to he noted, and the temperature of the ether has probably already the d...w-Imoint, one waits a minute until the dew on the polished tube evaporates and disappears. when another reading of the ther mometer is made. The mean of these two read ings should give the temperature of the de• point. to within a tenth of a degree.
When dew is thus deposited it indicates that the air from which the moisture comes Int, been eooled what is known as the point of satura tion and all the vapor associaled with the air can no hunger remain as vapor. This cooling Pr"ee" takes place naturally every clear night, so that in the rouse of the early morning not only are the grass and other exposed objects often covered wall dew, hut the quiet layers of the air near the ground b•..zin to condense their moisture in the
of deposition of dew, and espe eia11c the formation of fog, retards the cooling of leaves and other objects mai the ground and delay- the occurrence of freezing temperatures and in -I-. The actual quantity .4 dew lilting eti the :aid dripping to the ground, so as to I...Conic available fur the use I if the roots of the plant-. is ilk cloudless regions. such as Ariz..na, l.tlifornia, Syria, and -Nrabia, so large as to an important matter in agriculture.
lanY 'wen nntil" t" devise npp.tratus for measuring the absolute quantity of dew per unit of area of surface, hut these drostimeters lq.v i almost necessarily employ as the collect ing surface something different from the natural leave-, grass. and stones, and their measurements are at best not absolutely, but only relatively redrcsentative of t be natural phenomenon.
The discovery of the true explanation of the formation of dew is etonnionly ascribed to Dr. \V. c. If Cha ties•ton. S. (.. and London. published his new but is equally due to Alussehenbroek, Peclet. son. and others. The air that lies near the sur of the ground receives its moisture to a considerable extent from the soil lwileath it. and therefore only a small part of the dew from a great distance. The ground is perpetu ally either absorbing or gking out moisture. and the nearer the bedewed surface is to the ground the inure of this fresh moisture it will contain, so that the idea that dew falls from above, whence its Spanish name, sere 1M, and its French name. sm re in, nut correct, as was shown by Thassehenbroel: and \Veils, and very recently re stated by Stockbridge. The formation of dew and fog in night can only take place when land therefore indieates that ) the sky is suffi ciently cloudless to allow of almost uninter rupted radiation of heat. l'onsequently when any object is covered by even the slightest screen, is found on it.