If the dew-point is below the temperature of freezing, then, instead oI kle1N, tie haVe :1 forma tion frost. \Viten the wind is blowing. dew is not formed in dry weather. because the wind a supply of relatively warm, dry air to the cooling of the leaves and Ivarins them iip, by conduction. enough to prevent them from cooling to the dew-mint. But in Moist insular elimates dew may form for a while lec tor.• prev:tils.
\Vlicn the air is exceedingly dry the process of radiation (counter:let...I. as it always is, in part by the eondliel ion and eonvection of heat I may not be sufficiently intense or proeced far enough to cool ordinary •-arfaces down to the : will, of course, steadily fall during the might. but the sun may rise and warm up the ground before dew is formed. This can occur my in dry air, unlikely, that in which the dew point is Very low. Under Ille most hiVorahle you lit ions in the dry climate of Arizona the temperature at the surfaee may thus be lowered in the of a night by as as twenty. thirty. or forty degrees, when the depression of the dew point below the temperature of the dry air is till :'renter, ste that on spelt a night no dew is reeo•tied.
The number of degn.(... hy which the dew•point i, 1 met than the temperature of air is known the complement of t den-point. in addi tion to the apparatus there other indirect methods of determining the dew-point, which are less exact. but frequently eon. venient. The apparatus most commonly used in such cases is the so-called dry and Wet bulb ther mometer, or psychrometer, which will be found deseribed under 11Y(.1:0111:Ttat.
l of Many details with regard to dew and cog nate phenomena consult the works of Tomlinson. especially The Dewdrop um/ rite .ffisf (London. ISti(1), and his historical article in the La' i Him h etr ilosoph lea/ Journal, vul. xiii. (Edinburgh, Istil ).
DEW, Tiloci.cs (1s02-46).
American educator and writer. lie was born in ling and 1...iteen l'ounty. •a., and graduated in 182.0 at 1Villiam and .1111'y riillege, where he was professor of metaphysics, and polit ical oeolionly from ls.2.7 to I S:{ti, and served as president from 1M30 until his death. In Iti3• he published a review of the celebrated debate 1S31-:3• mer slavery in the Virginia Legislature, under the title Essog in roror of Siorcry, which weal far toward putting a stop to a IlloVe Men!. then assuming considerable proportions. to proelaitti emancipation in Virginia. His most important. work is a /horst of La ms, Cus toms, Thrum rs, uml I its! itut ions of .1 neient and .11 ofb•ra at ions ( l`+53).
DEWALQUE. 1:11.1.us 1S211—l. A Belgian geologist. born at Stavelot. lie became curator of the mineralog ical and geological eabin•t at the University .4 Liige in IS55. and professor Illvre of Minor:do*. and paleontology in is..57. 17t) lie W:i. president 14 the Belgian .\catlemy. In addition to a considerable number of cunt rilm lions to seientitic periodleals, he huts published a De.,•ription ern l,ias duns I. 1.u.rt ?Omar!, 11857 1 ; an IIlus ele cristal logo: ph ( Pt-oilman, d' nue description .74'0/m/it/tie de la Belgique (2,1 IS743: and iiiiiet