DEW. 186 DEW ART. with dew, but the quii't layers of the air near the gruund begin to t-oiideiise their moisture in the shajie of fog. The deposition of dew, ami e.-pe- eially the formation of foj;, retards the cooling of leaves and other objects near the ground and delays the oceurrcnee of freezing temperatures and "frosts. The actual ipiaatily of dew accunui- lating on the leaves and dripping to the ground, so as to become available for the use of the roots of the plants, is in cloudless regions, such as Arizona, California. Syria, and Arabia, so large as to be an important matter in agriculture. Many attempts have been niaile to devise appa- ratus for measuring the absolute quantity of dew per unit of area of surface, but these drosometers (q.v.) almost necessarily employ as the collect- ing surface something dillerent from the natural leaves, grass, and stones, and their measurements are at best not absolutely, but only relatively representative of the natural phenomenon. The iliscovery of the true explanation of the formation of dew is conunonly ivscribed to Dr. V. C. Wells, of Charleston. .S. C, and London. Kng., who published his Esxai/ on Dew in 1S14, but is equally due to Musschenbroek, Peclet, Wil- son, and others. The air that lies near the sur- face of the ground receives its moisture to a considerable extent from the soil beneath it. and therefore only a small part of the dew comes from a great distance. The ground is perpetu- ally either absorbing or giving out moisture, and the nearer the bedewed surface is to the ground the more of this fre~h moisture it will contain, so that the idea that dew falls from above, whence its Spanish name, sercmi. and its French name, serein, is not correct, as was shown by Musschenbroek and Wells, and very recently re- stated by Stockbridge. The formation of dew and fog in the night can only take idace when (and therefore indicates that) the sky is suiri- ciently cloudless to allow of almost uninter- rupted radiation of heat. Consequently when any object is covered by even the slightest screen, no dew is found on it. If the dew-point is below the temperature of freezing, then, instead of dew. we have a forma- tion of frost. When the wind is blowing. dev is not formed in dry weather. because the wind brings a supply of relatively warm, dry air to the cooling surfaces of the leaves and warms them up, by conduction, enough to prevent them from cooliiig to the dew-point. But in moist insular climates dew nuiy form for a while be- fore fog prevails. When the air is exceedingly dry the process of radiation (counteracted, as it always is, in part by the conduction and convection of heat) may not be sufliciently intense or proceed far enough to cool ordinary surfaces down to the dcw-iioint : temperatures will, of coiirse, steadily fall during the night, but the sun may rise and warm up the ground before dew is formed. This can occur only in dry air, namely, that in which the dew- point is very low. I'niler the most favorable conditions in the dry climate of Arizona the temperature at the surface may thus be lowered in the course of a night lv as much as twenty, thirty, or forty degrees, when the depression of the dew-point below the temperature of the dry air is still greater, so that on such a night no dew is recorded. The number of degrees by which the dew-point is lower than the teniperatire of the air is known as the complcmrnt of the deu-iioinl. In addi- tion to the tlew-point apparatus there are other indirect methods of determining the dew-point, which are less e.act. but frciiucnlly more con- venient. The apparatus most ci>nunonly used in such eases is the so-called dry and wet bulb ther- mometer, or psychromelcr. which will be found described lUider llYuRdMKTKlt. For many details with regard to dew and cog- nate phenomena consult the works of Charles Tomlinson. especially The Ocici/roy) uiul the Mist (I.ondim. IS(iO), and his historical article in the Kfliiibiinih .Y( ir I'hilosoiihieal Journal, vol. xiii. (Edinburgh, 1801). DEW, Thomas Rodkuick (1802-40). -
. icrican educator and writer. He was born in King and Qieen County, Va., ami graduated in 1820 at William and .Mary College, where he was professor of history, metajjliysics. and polit- ical economy from 1827 to 18;{0, and served as president from 1830 until his death. In 18.'J2 he piblished a review of the celebrated debate of 1831-32 over slavery in the Virginia Legislature, under the title An Kssiiii in I'tnur of Slarerii. which went far toward putting a stop to a move- ment, then assuming considerable proportions, to proclaim emancipation in Virginia. His most important work is a Difiest of the Laws, Ciis- lows. Manners, and Instilulions of Ancient ami Modern ations (1853). DEWALQUE, cb-valk'. Giu.es .Joseph Gis- TAK ( is2(i — I. . Helgian geologist, born at Stavelot. He became curator of the mineralog- ical and geological cabinet at the University of Li{>ge in 18.55, and professor there of mineralogy, geology, and paleontology in 1857. In 1870 he was elected president of the Helgian .eademy. In addition to a considerable numlier of contribu- tions to seientilic jieriodieals. he has piiblished a Description dti Lias dans le Lnj-emhourfl ( 1857) ; an Atlas.de cristallo<iraiihie (1800); I'rodrome d'une description rieoloiii'iiie de la liclgiquc (2il ed. 1874) ; and other works. DEWAR, .LMES (1842—). A British chem- ist, born at Kincardineon-Forth, Scotland. He was educated at Edinburgh I'niversity, and w'as assistant there to Lord Playfair when he occui)ied the chair of chemistry. Subsc(]Uently be became .Jacksonian professor of experimental philosophy at Cambridge, and FuUerian professor of ebcm- istry in the Royal Institution. In 1807 he was elected president of the Chemical .Soi'iety. and in 1002 of the Hrilish .ssociation. His scientific researches include studies in regard to the physiological action of light and the licpiefaction of gases, and in particular investigations of low temixM-alures. Hy means of the evaporatiim of liipiid hydrogen under a reduced atmosphcri<' pressure he obtained the lowest temiMTature yet reached —2.50° C. ( — 403° F.). With Sir Fred- evick Abel (q.v.) he invented cordite, a smoke- less gunpowder used by the British Government. DEW' ART, EowAiii) IlAitTiEV (1828—). Canadian elergvnuin and author, born in C(mnty Cavan. Ireland. He was educated in Toronto, Canada, and was called to the WesU-van Metho- dist ministry in 1851. He edited (1800 04) the Christian Cunrdian. the chief organ of his Church in Canada, and published, besides other work.?, itimps of Life and Selections from the Canadian Poets.
