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water it is called hoarfrost. The researches of Aitken suggest that the words " by condensation of the vapour in the atmosphere " might be omitted from the See also:

DEFINITION (Lat. definitio, from de-finire, to set limits to, describe)

definition. He has given reasons for believing that the large dewdrops on the leaves of See also:

PLANTS

plants, the most characteristic of all the phenomena of dew, are to be accounted for, in large measure at least, by the exuding of drops of water from the plant through the pores of the leaves themselves. The formation of dewdrops in such cases is the continuation of the See also:

IRRIGATION (Lat. in, and rigare, to water or wet)

irrigation See also:

PROCESS

process of the plant for supplying the leaves with water from the See also:

SOIL

soil. ' The process is set up in full vigour in the daytime to maintain tolerable thermal conditions at the surface of the See also:

LEAF (O. Eng. leaf, cf. Dutch loof, Ger. Laub, Swed. lof, &c.; possibly to be referred to the root seen in Gr. Ma-eta, to peel, strip)

leaf in the hot See also:

sun, and continued after the sun has gone. On the other See also:

hand, the most typical See also:

PHYSICAL

physical experiment illustrating the formation of dew is the production of a deposit of moisture, in minute drops, upon the exterior surface of a See also:

glass or polished See also:

metal See also:

VESSEL (O. Fr. vaissel, from a rare Lat. vascellum, dim. of vas, vase, urn)

vessel by the cooling of a liquid contained in the vessel. If the liquid is water, it can be cooled by pieces of ice; if volatile like See also:

ETHER, (C2H5)2O

ether, by bubbling See also:

air through it.

No deposit is formed by this process until the temperature is reduced to a point which, from that circumstance, has received a See also:

SPECIAL

special name, although it depends upon the See also:

state of the air See also:

ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

round the vessel. So generally accepted is the physical See also:

ANALOGY (Gr. avaXo-yLa, proportion)

analogy between the natural formation of dew and its artificial production in the manner described, that the point below which the temperature of a surface must be reduced in See also:

order to obtain the deposit is known as the " dew-point." In the view of physicists the dew-point is the temperature at which, by being cooled without See also:

CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change of pressure, the air becomes saturated with water vapour, not on See also:

account of any increase- of See also:

SUPPLY (through Fr. from Lat. supplere, to fill up)

supply of that See also:

compound, but by the diminution of the capacity of the air for holding it in the gaseous See also:

CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

condition. Thus, when the dew-point temperature has been determined, the pressure of water vapour in the atmosphere at the See also:

time of the deposit is given by reference to a See also:

TAROK

tarok of saturation pressures of water vapour at different temperatures. As it is a well-established proposition that the pressure of the water vapour in the air does not vary while the air is being cooled without change of its See also:

TOTAL

total See also:

EXTERNAL

external pressure, the saturation pressure at the - dew-point gives the pressure of water vapour in the air when the cooling commenced. Thus the artificial formation of dew and consequent determination of the dew-point is a recognized method of measuring the pressure, and thence the amount of water vapour in the atmosphere. The dew-point method is indeed in some ways a fundamental method of hygrometry. - The dew-point is a See also:

MATTER

matter of really vital consequence in the question of the oppressiveness of the atmosphere or its See also:

REVERSE

reverse. So See also:

long as the dew-point is See also:

low, high temperature does not matter, but when the dew-point begins to approach the normal temperature of the human See also:

BODY

body the atmosphere becomes insupportable. The physical explanation of the formation of dew consists practically in determining the process or processes by which leaves, See also:

BLADES, WILLIAM (1824-1890)

blades of grass, stones, and other See also:

OBJECTS

objects in the open air upon which dew may be observed, become cooled " below the dew-point." Formerly, from the time of See also:

Aristotle a-least, dew was supposed to " fall." That view of the process was not See also:

EXTINCT

extinct at the time of See also:

Wordsworth and poets might ever now use the figure without reproach. To Dr See also:

Charles See also:

Wells of See also:

LONDON

London belongs the See also:

CREDIT (Lat. credere, to believe)

credit of bringing to a See also:

FOCUS (Latin for " hearth " or " fireplace ")

focus the ideas which originated with the study of See also:

RADIATION, THEORY OF

radiation at the beginning of the loth See also:

CENTURY (from Lat. centuria, a division of a hundred men)

century, and which are expressed by saying that the cooling necessary to produce dew on exposed surfaces is to be attributed to the radiation from the surfaces to a clear See also:

SKY (M. Eng. skie, cloud; O. Eng. skua, shade; connected with an Indo-European root sku, cover, whence " scum," Lat. obscurus, dark, &c.)

sky. He gave an account of the theory of automatic cooling by radiation, which has found a See also:

PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place in all See also:

TEXT (Lat. textum, woven fabric, from texere, to weave)

text-books of physics, in his first See also:

ESSAY, ESSAYIST (Fr. essai, Late Lat. exagium, a weighing or balance; exigere, to examine; the term in general meaning any trial or effort)

Essay on Dew published in 1818. The theory is supported in that and in a second essay by a number of well-planned observations, and the essays are indeed See also:

MODELS

models of scientific method.

The process of the formation of dew as represented by Wells is a See also:

SIMPLE

simple one. It starts from the point of view that all bodies are constantly radiating See also:

HEAT (0. E. haktu, which like " hot," Old Eng. hat, is from the Teutonic type haita, hit, to be hot; cf. Ger. hitze, heiss; Dutch, hitte, heet, &c.)

heat, and cool automatically unless they receive a corresponding amount of heat from other bodies by radiation or See also:

CONDUCTION, ELECTRIC

conduction. See also:

GOOD, JOHN MASON (1764-1827)

Good radiators, which are at the same time See also:

BAD, BCD

bad conductors of heat, such as blades of grass, lose heat rapidly on a clear night by radiation to the sky and become cooled below the dew-point of the atmosphere. The question was very fully studied by See also:

MELLONI, MACEDONIO (1798-1854)

Melloni and others, but little more was added to the explanation given by Wells until 1885, when See also:

John Aitken of See also:

FALKIRK

Falkirk called See also:

ATTENTION (from Lat. ad-tendo, await, expect; the condition of being " stretched " or " tense ")

attention to the question whether the water of dewdrops on plants or stones came from the air or the See also:

earth, and described a number of experiments to show that under the conditions of observation in See also:

Scotland, it was the earth from which the moisture was probably obtained, either by the operation of the vascular See also:

SYSTEM

system of plants in the formation of exuded dewdrops, or by evaporation and subsequent condensation in the lowest layer of the atmosphere. Some controversy was excited by the publication of Aitken's views, and it is interesting to revert to it because it illustrates a proposition which is of See also:

general application in meteorological questions, namely, that the physical processes operative in the See also:

EVOLUTION

evolution of meteorological phenomena are generally complex. It is not radiation alone that is necessary to produce dew, nor even radiation from a body which does not conduct heat. The body must be surrounded by an atmosphere so fully supplied with moisture that the dew-point can be passed by the cooling due to radiation. Thus the conditions favourable for the formation of dew are (1) a good radiating surface, (2) a still atmosphere, (3) a clear sky, (4) thermal insulation of the radiating surface, (5) warm moist ground or some other See also:

PROVISION (Lat. provisio)

provision to produce a supply of moisture in the surface layers of air. Aitken's contribution to the theory of dew shows that in considering the supply of moisture we must take into See also:

CONSIDERATION (from Lat. considerare, to look at closely, examine, generally taken to be from con-, and the base seen in sidus, sideris, a star, the word being supposed to be originally an astrological or astronomical term)

consideration the ground as well as the air and concern ourselves with the temperature of both. Of the five conditions mentioned, the first four may be considered necessary, but the fifth is very important for securing a copious deposit. It can hardly be maintained that no dew could form unless there were a supply of water by evaporation from warm ground, but, when such a supply is forthcoming, it is evident that in place of the limited process of condensation which deprives the air of its moisture and is therefore soon terminable, we have the process of See also:

DISTILLATION (from the Lat. distillare, more correctly destillare, to drop or trickle down)

distillation which goes on as long as conditions are maintained. This distinction is of some See also:

PRACTICAL

practical importance for it indicates the protecting See also:

POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power of wet soil in favour of See also:

young plants as against night See also:

frost.

If distillation between the ground and the leaves is set up, the temperature of the leaves cannot fall much below the See also:

ORIGINAL

original dew-point because the supply of water for condensation is kept up; but if the See also:

COMPENSATION (from Lat. compensare, to weigh one thing against another)

compensation for loss of heat by radiation is dependent simply on the condensation of water from the atmosphere, without renewal of the supply, the dew-point will gradually get See also:

LOWER

lower as the moisture is deposited and the process of cooling will go on. In these questions we have to See also:

DEAL

deal with comparatively large changes taking place within a small range of level. It is with the layer a few inches thick on either See also:

side of the surface that we are principally concerned, and for an adequate comprehension of the conditions See also:

close consideration is required. To illustrate this point reference may be made to See also:

FIGS

figs. 1 and 2, which represent the condition of affairs at Io•40 P.M. On about the loth of See also:

OCTOBER

October 1885, according to observations by Aitken. See also:

VERTICAL (from Lat. vertex, highest point)

Vertical distances represent heights in feet, while the temperatures of the air andthe dew-point are represented by See also:

HORIZONTAL

horizontal distances and their See also:

variations with height by the curved lines of the See also:

DIAGRAM

diagram. The See also:

LINE

line marked o is the ground level itself, a rather indefinite quantity when the surface is grass. The whole vertical distance represented is from 4 ft. above ground to i ft. below ground, and the special phenomena which we are considering take place in the layer which represents the rapid transition between the temperature of the ground 3 in. below the surface and that of the air a few inches above ground. The point of See also:

INTEREST

interest is to determine where the dew-point See also:

CURVE (Lat. curvus, bent)

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 See also:

YEAR

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 See also:

ESCAPE (in mid. Eng. eschape or escape, from the O. Fr. eschapper, modern echapper, and escaper, low Lat. escapium, from ex, out of, and cappa, cape, cloak; cf. for the sense development the Gr. iichueoOat, literally to put off one's clothes, hence to sli

escape, the moisture cannot. Thus the dry-bulb line is deflected to the See also:

LEFT

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 See also:

case of grass, fig. 2, the deviation of the dry-bulb line to the left to form a See also:

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 See also:

GRADUAL (Med. Lat. gradualis, of or belonging to steps or degrees; gradus, step)

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 See also:

LOANGO

Loango See also:

COAST (from Lat. costa, a rib, side)

coast at 3 mm., but the estimate seems a high one. Measurements go to show that the See also:

DEPTH

depth of water corresponding with the aggregate See also:

ANNUAL

annual deposit of dew is 1 in. to r•5 in. near London (G. Dines), 1.2 in. at See also:

MUNICH (Ger. Munchen)

Munich (Wollny), o•3 in. at See also:

MONTPELLIER

Montpellier (Crova), 1•6 in. at See also:

TENBURY

Tenbury, See also:

WORCESTERSHIRE

Worcestershire (Badgley). With the question of the amount of water collected as dew, that of the See also:

MAINTENANCE (Fr. maintenance, from maintenir, to maintain, support, Lat. manu tenere, to hold in the hand)

maintenance of " dew ponds " is intimately associated. The name is given to certain isolated ponds on the upper levels of the See also:

CHALK

chalk See also:

DOWNS

downs of the See also:

south of See also:

England and elsewhere. Some of these ponds are very ancient, as the See also:

TITLE (0. Fr. title, mod. titre, from Lat. titulus)

title of a See also:

WORK

work on See also:

NEOLITHIC, or LATER STONE AGE (Gr. vEOr, new, and XLOos, stone)

Neolithic Dewponds by A. J. and G. Hubbard indicates. Their name seems to imply the See also:

HYPOTHESIS (from Gr. inrorcOivat, to put under; cf. Lat. suppositio, from sub-ponere)

hypothesis that they depend upon dew and not entirely upon See also:

RAIN (O.E. regn; the word is common to Teutonic languages, cf. Ger. Regen, Swed. and Dan. regn; it has been connected with Lat. rigare, to wet, Gr. 13pixeav)

rain for their maintenance as a source of water supply for See also:

CATTLE (Norman Fr. catel, from Late Lat. capitale, wealth or property, a word applied in the feudal system to movable property and particularly to live stock, and surviving in its wider meaning as " chattel " or " chattle ")

cattle, for which they are used. The question has been discussed a good deal, but not settled; the See also:

BALANCE (derived through the Fr. from the Late Lat. bilantia, an apparatus for weighing, from bi, two, and lanx, a dish or scale)

balance of See also:

EVIDENCE (Lat. evidentia, evideri, to appear clearly)

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 See also:

pond has first to be filled artificially. Grass See also:

EAR (common Teut.; O.E. are, Ger. Ohr, Du. oor, akin to Lat. auris, Gr. ovs)

Ear emper 80° 40° sor It does not come' into existence by the gradual See also:

ACCUMULATION (from Lat. accumulare, to heap up)

accumulation of water in an impervious See also:

basin.

For Dew Ponds, see See also:

Stephen See also:

Hales, Statical Essays, vol. i., experiment xix., pp. 52-57 (2nd ed., London, 1731) ; See also:

Gilbert See also:

White, Natural See also:

HISTORY

History and Antiquities of See also:

Selborne, See also:

LETTER (through Fr. lettre from Lat. littera or litera, letter of the alphabet; the origin of the Latin word is obscure; it has probably no connexion with the root of linere, to smear, i.e. with wax, for an inscription with a stilus)

letter See also:

XXIX

xxix. (London, 1789) ; Dr C. Wells, An Essay on Dew (London, 1818, 1821 and 1866) ; Rev. J. C. Clutterbuck, " See also:

PRIZE, or PRIZE OF WAR (Fr. prise, from prendre, to take)

Prize Essay on Water Supply," Journ. See also:

ROY, WILLIAM (1726-1790)

Roy. Agric. See also:

Soc., 2nd See also:

SERIES (a Latin word from serere, to join)

series, vol. i. pp. 271-287 (1865); See also:

Field and See also:

Symons, " Evaporation from the Surface of Water," Brit. Assoc.

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