Fig. 1.Fig. 2.

The point of interest is to determine where the dew-point curve and dry-bulb curve will cut. If they cut above the surface, mist will result; if they cut at the surface, dew will be formed. Below the surface, it may be assumed that the air is saturated with moisture and any difference in temperature of the dew-point is accompanied by distillation. It may be remarked, by the way, that such distillation between soil layers of different temperatures must be productive of the transference of large quantities of water between different levels in the soil either upward or downward according to the time of year.

These diagrams illustrate the importance of the warmth and moisture of the ground in the phenomena which have been considered. From the surface there is a continual loss of heat going on by radiation and a continual supply of warmth and moisture from below. But while the heat can escape, the moisture cannot. Thus the dry-bulb line is deflected to the left as it approaches the surface, the dew-point line to the right. Thus the effect of the moisture of the ground is to cause the lines to approach. In the case of grass, fig. 2, the deviation of the dry-bulb line to the left to form a sharp minimum of temperature at the surface is well shown. The dew-point line is also shown diverted to the left to the same point as the dry-bulb; but that could only happen if there were so copious a condensation from the atmosphere as actually to make the air drier at the surface than up above. In diagram 1, for soil, the effect on air temperature and moisture is shown; the two lines converge to cut at the surface where a dew deposit will be formed. Along the underground line there must be a gradual creeping of heat and moisture towards the surface by distillation, the more rapid the greater the temperature gradient.

The amount of dew deposited is considerable, and, in tropical countries, is sometimes sufficiently heavy to be collected by gutters and spouts, but it is not generally regarded as a large percentage of the total rainfall. Loesche estimates the amount of dew for a single night on the Loango coast at 3 mm., but the estimate seems a high one. Measurements go to show that the depth of water corresponding with the aggregate annual deposit of dew is 1 in. to 1.5 in. near London (G. Dines), 1.2 in. at Munich (Wollny), 0.3 in. at Montpellier (Crova), 1.6 in. at Tenbury, Worcestershire (Badgley).

With the question of the amount of water collected as dew, that of the maintenance of "dew ponds" is intimately associated. The name is given to certain isolated ponds on the upper levels of the chalk downs of the south of England and elsewhere. Some of these ponds are very ancient, as the title of a work on Neolithic Dewponds by A. J. and G. Hubbard indicates. Their name seems to imply the hypothesis that they depend upon dew and not entirely upon rain for their maintenance as a source of water supply for cattle, for which they are used. The question has been discussed a good deal, but not settled; the balance of evidence seems to be against the view that dew deposits make any important contribution to the supply of water. The construction of dew ponds is, however, still practised on traditional lines, and it is said that a new dew pond has first to be filled artificially. It does not come into existence by the gradual accumulation of water in an impervious basin.

Authorities.—For Dew, see the two essays by Dr Charles Wells (London, 1818), also "An Essay on Dew," edited by Casella (London, 1866), Longmans', with additions by Strachan; Melloni, Pogg. Ann. lxxi. pp. 416, 424 and lxxiii. p. 467; Jamin, "Compléments à la théorie de la rosée," Journal de physique, viii. p. 41; J. Aitken, on "Dew," Trans. Roy. Soc. of Edinburgh, xxxiii., part i. 2, and "Nature," vol. xxxiii. p. 256; C. Tomlinson, "Remarks on a new Theory of Dew," Phil. Mag. (1886), 5th series, vol. 21, p. 483 and vol. 22, p. 270; Russell, Nature, vol 47, p. 210; also Met. Zeit. (1893), p. 390; Homén, Bodenphysikalische und meteorologische Beobachtungen (Berlin, 1894), iii.; Taubildung, p. 88, &c.; Rubenson, "Die Temperatur-und Feuchtigkeitsverhältnisse in den unteren Luftschichten bei der Taubildung," Met. Zeit. xi. (1876), p. 65; H. E. Hamberg, "Température et humidité de l'air à différentes hauteurs à Upsal," Soc. R. des sciences d'Upsal (1876); review in Met. Zeit. xii. (1877), p. 105.

For Dew Ponds, see Stephen Hales, Statical Essays, vol. i., experiment xix., pp. 52-57 (2nd ed., London, 1731); Gilbert White, Natural History and Antiquities of Selborne, letter xxix. (London, 1789); Dr C. Wells, An Essay on Dew (London, 1818, 1821 and 1866); Rev. J. C. Clutterbuck, "Prize Essay on Water Supply," Journ. Roy. Agric. Soc., 2nd series, vol. i. pp. 271-287 (1865); Field and Symons, "Evaporation from the Surface of Water," Brit. Assoc. Rep. (1869), sect., pp. 25, 26; J. Lucas, "Hydrogeology: One of the Developments of Modern Practical Geology," Trans. Inst. Surveyors, vol. ix. pp. 153-232 (1877); H. P. Slade, "A Short Practical Treatise on Dew Ponds" (London, 1877); Clement Reid, "The Natural History of Isolated Ponds," Trans. Norfolk and Norwich Naturalists' Society, vol. v. pp. 272-286 (1892); Professor G. S. Brady, On the Nature and Origin of Freshwater Faunas (1899); Professor L. C. Miall, "Dew Ponds," Reports of the British Association (Bradford Meeting, 1900), pp. 579-585; A. J. and G. Hubbard, "Neolithic Dewponds and Cattle-Ways" (London, 1904, 1907).

(W. N. S.)

DEWAN or Diwan, an Oriental term for finance minister. The word is derived from the Arabian diwan, and is commonly used in India to denote a minister of the Mogul government, or in modern days the prime minister of a native state. It was in the former sense that the grant of the dewanny to the East India Company in 1765 became the foundation of the British empire in India.