December 29, 1892] 



NATURE 



211 



therefore more dew would be deposited from the air ; but this 

 objection would scarcely be valid where a small plant was in- 

 closed on bare earth and the deposition on the interior of the 

 glass compared with that on a glass not inclosing a plant. 



Recent investigations have proved the evaporation from plants 

 to be very large, and since evaporation proceeds by night as 

 well as by day, there can be no reason why a moderate pro- 

 portion of the dew deposited on the surface of blades of grass 

 and on leaves of plants generally should not be derived from the 

 vapour which they exhale. The fact that an equal quantity of 

 dew is deposited on glass, china, painted wood, &c., exposed to 

 the sky to that depo-ited on grass, may seem to minimize the 

 influence of plant exhalation, but we must remember that the 

 whole of the stratum of air near the ground is rendered more 

 vaporous by these exhalations, and that therefore the dew-point 

 is sooner reached on the surface of any body exposed to the sky 

 in the midst of vege'auon than on bare open ground. Moreover, 

 the thickness of ihe substance prevents earth heat from much 

 affecting the upper surface. The effsct of grass in promoting 

 dew formation is owing — (i) To its radiative power cooling its 

 surface below the dew-point. (2) To the consequent c loling of 

 the stratum of air in and over the grass to a point much below 

 that of the air a few feet higher. (3) To the obstruction offered 

 by the grass to any light air or breeze on a nearly calm night, 

 and the consequent settling down, without much disturbance, of 

 a cold heavy stratum. {4) To the prevention by the grassy 

 covering of the drying up process by sun and wind which takes 

 place on bare ijround, and to the moist earth which therefore 

 exists under grass near the surface even in dry weather. {5) To 

 the exhalation of vapour from the grass. 



The realization of these causes explained what was always, 

 previous to these observations, a difficulty to me, the almost 

 entire absence of dew on heather and dry fern in the summer. 

 Even after heavy dews, heather was invariahly found perfectly 

 dry. In fine, calm winter weather, with white frost, heather 

 may be a good deal whitened, and the frost is then derived 

 largely frim the open air. Wood, being a good radiator and 

 bad conductor, is heavily bedewed and frosted. 



Stones, whether of sandy composition and appearance, or of 

 close grain like flint, pebbles, and slate, are not often visibly 

 bedewed or frosted on the top on clear nights. On their sur- 

 faces, touching or very close to the ground, they are heavily be- 

 dewed and frosted. A moderate radiative power, their usual 

 situation removed from grass ani vegetation, and in the case of 

 the close grained stones, a conductive power greater than that 

 of leaves, grass, and wood, though less than that of metals, pre- 

 vent the deposition of much atmospheric moisture on their 

 exposed sides. But when air highly charged with vapour im- 

 pinges on them in a confined space, as on their lower sides, con- 

 densation readily takes place, just as it will take place when any 

 substance, even polished metal, is held above the spout of a 

 kettle of boiling water. It is apparent that since stones act as 

 condensors to the vapour constantly arising from the earth, and 

 since the heat of the sun and temperature of the air by day only 

 slightly raise the temperature of the earth immediately beneath 

 a large stone, while the radiation of heat from the stone and low 

 air temperature of the night cause the lower side of the stone to 

 be very cold at night, a rather large amount of moisture must be 

 deposited on its lower surface in every twenty-four hours, and the 

 ground on which it rests must in our climate remain always very 

 moist. The space between the stone and the ground conse- 

 quently becomes the abode of many insects which live well in 

 damp and darkness. 



Occasional observation of the distribution of dew, without 

 careful comparison with the state of the weather, gives an im- 

 pression of capriciousness which only continuous records com- 

 prising various conditions can remove. 



Deposition is generally favoured by a humid air, and therefore 

 in this country by southwesterly and westerly winds, which bring 

 over the land the vapour derived fromevaporationof the Atlantic 

 Ocean. A smaller fall of temperature by radiation brings about 

 condensation, and there is less tendency in any dep>sit to 

 evaporate than in a drier air. Radiation may produce a greater 

 fall of temperature in dry air, but the distance from the dew- 

 point is commonly too wide to compensate greater humidity 

 with greater cooling. 



Calm is also very favourable to dew-formation. It allows 

 parcels of vapour in the air to remain sufficiently long in contact 

 with cold radiating substances to become greatly cooled, and so 

 to become condensed upon them, and it prevents the dispersion | 



of the stratum of air near the ground, which is continually cool- 

 ing by contact and radiation. Thus dew goes on forming while 

 the air falls lower and lower beyond its original dew point, and 

 while by a very gentle movement an interchange is kept upbe- 

 tween the warmer air touching the ground beneath the grass, and 

 the cold air on the surface of the grass, and between differently 

 cooled layers and portions of air above it. If the air is very 

 humid, a very slight air or breeze is favourable to heavy de- 

 position. On ordinary clear nights, calm and light airs allow 

 the reduction of the lowest stra'um of air to the dew-point, and 

 there is no liability to evaporation of the minute depo^^ited par- 

 ticles by portions of air above the dew-point being driven against 

 them. When the air is rather dry, as often happens at night in 

 dry summer weather, and in winter frosts, calm is frequently a 

 necessary condition for the deposit and appearance of dew and 

 white frost. The deposit may be observed to take place on the 

 cessation of wind, and again, the change from calm to wind soon 

 dries off the dew which has already formed. On other occasions, 

 when there is a gentle air or breeze, dew and frost are deposited 

 only in sheltered places, as on the most sheltered slopes of fields, 

 on banks sloping to leeward, on leaves on the lee side of bushes 

 and trees, on the lee side of mole-hills, posts, railings, and other 

 objects. Hollows, depressions, and cracks, in paper, glass, 

 stones, tiles, wood, and leaves, are more bedewed than flat 

 surfaces from the same reason, — the reduction below the dew- 

 point of air less diluted than thit which is more free by currents 

 of higher temperature and greater dryness. With a fresh west 

 wind in a clear night, the raised and ribbed parts of leave'?, &c., 

 may be thickly bedewed and frosted, but the hollows and folds 

 scarcely if at all less, and the sides of buds, thorns, &c., are 

 more frosted than the points. The wind is, in fact, often 

 sufficiently removed from the dew-point to prevent deposition 

 or continuance of moisture on all parts which are fully exposed 

 to it. Not even free radiation to a clear sky then avails 

 to plant frost-growths upon the object whose temperature is 

 being perpetually supplied by the forcible impact of warmer air. 



Free radiation or exposed situation is, on the whole, perhaps 

 the most effectual cause of dew on very many nights in the year. 

 In a level country those parts of a field which are least sheltered 

 by trees and hedges gather most dew and frost on calm nights. 

 Similarly, those parts of any flat sub-;tance, such as a sheet of 

 glass or paper, which have the most uninterrupted exposure to 

 the sky are most bedewed. The tops of bushes, posts, railings, 

 inverted drinking glasses, pans, &c., are on calm nights, and 

 sometimes breezy nights, more bedewed than the sides. Greater 

 cold by greater radiation in these cases produces greater deposition 

 from the cooled air which comes in contact with the freely radiat- 

 ing surfaces. It must be remarked, however, that radiation from 

 fine points, such as the tips of sharp thorns, is often not sufficient 

 to counteract in air which is not very humid the effect of the 

 continuaf impact of air above the dew-point and higher in tem- 

 perature. Close to the ground the case is different, for there the 

 temperature of the low stratum of air is lower, and usually about 

 the dew-point, there is little movement, and vapour from the 

 ground increases humidity ; but even in this situation the points 

 of grasses, &c., are often less bedewed than the sides. 



That free radiation is by no means necessary for the formation 

 of heavy dew on grass is proved by the experiments detailed 

 above, made during the summer of 1892. The grass was found 

 heavily bedewed in dry weather within three enclosures of 

 earthenware by which radiation was arrested. 



Since grass covered by hollow vessels, and the interior of 

 hollow vessels themselves, are thickly covered with dew, it would 

 seem likely that the grass under overhanging trees would be as 

 thickly bedewed as the exposed grass in a field, and that the 

 undersides of the overhanging leaves would also be wetted. 

 This is not the case. And there are differences in the two 

 situations sufficient to account for the absence of dew under 

 leafy trees. In the first place, on a calm night, the air under a 

 tree is warmer than in the open owing to radiation from the 

 ground being arrested. Secondly, whatever vapour escapes from 

 the earth is unable to condense on the grass which covers it, the 

 grass being but little colder than the air and vapour. Thirdly, 

 and herein lies the chief difference, the air under the tree moves 

 freely and is above the dew-point, since the earth and other 

 objects which it touches are warmer than the grass and air out- 

 side. If the air were confined in a small space, the increments 

 of vapour issuing from the earth, and the gradual cooling of the 

 grass under the tree and of the tree itself, might cause deposition, 

 but air which has parted with much of its moisture outside is 



NO. 1209, VOL. 47] 



