NA TURE 



[December 29, 1892 



constani ly mixing witii a considerable b Ddy of air already warmed 

 under the shelterinjj canopy. Thus all objects under' the tree 

 remain above or not much below the dew-point of the air which 

 touches tliem. Yet, on a calm nij>ht, long grass and other sub- 

 stances a little raised above the ground are sometimes heavily 

 bedewed, though largely hindered by overhanging branches from 

 losing their heat by radiation. They often remain nearly dry 

 till the morning hours, and then reach a temperature below the 

 dew-point. The absence of dew under trees and bushes is, within 

 limits, roughly proportional to the area of ground covered. A 

 large surface of dry ground slowly parting with its heat during 

 the night has a powerful effect in preventing condensation. 

 Small bushes on a humid clear night are often much bedewed 

 even on their lower leaves. On the night of October 5, 1892, 

 both sides of the leaves of bushes in all sheltered situations were 

 found thickly bedewed, but where leaves were either exposed 

 to the slight breeze which was blowing, or near the wall of the 

 house on which the sun had shone, they were dry. The warm, dry 

 wall of a house acts a part similar to that of the earth under a 

 tree in radiating warmth to neighbouring objects, and in warming 

 the air by contact. The vapour emerging Irom earth sheltered 

 by foliage several feet above it has time to mix well with air 

 before coming in contact with solid objects. In the hollow 

 vessels, and even in the space between a raised plate of glass and 

 the earth, the vapour which rises from the earth has no lime to 

 become equally distributed in the air before meeting with sub- 

 stances colder than itself ; in the closed ve>sel> the initial amount 

 of vapour is augmented so as to produce constant saturation. 

 Objects, such asdrinking-glasses, raised several feet above the 

 grass, were seldom muchbeaewed, and often quite dry. 



The increase of pasture-land in England must have a con- 

 siderable effect in increasing cold by radiation, and in diminish- 

 ing the amount of vapour in the air at night by deposition on 

 grass. The sensible moisture at night must be increased near 

 the ground, the dew-point being quickly reached on a clear 

 night over grass. 



The large quantity of dew found on plates and other objects 

 over sandy ground, dry to a depth of several inches, proves the 

 possibility of a large emanation of noxious vapours from soil 

 containing decaying organic matter below a covering of sand. 

 The ague of parts of East Anglia and of sandy malarious districts 

 may be thus accounted for. 



Houses built on sandy ground over a damp subsoil may be 

 considered as scarcely more wholesome than if built on the damp 

 soil itself. 



In late summer ani early autumn the high temperature of the 

 soil in comparison with the temperature of the surface and of the 

 air near the ground at night, must have a powerful effect in the 

 production of vaporous exhalations. The heavy rains which so 

 often occur in October, the wettest month of the year< must co- 

 operate with a falling air-temperature in driving out air from 

 the pores of the earth. 



In nearly all the conclusions of Wells, as stated in his ad- 

 mirable " Essay on Dew," my observations lead me to concur. 

 He found that calm is favourable to the precipitation of dew ; 

 that if, in the course of the right, the weather, from being calm 

 and serene, became windy and cloudy, not only did dew cease to 

 form, but that which had formed either disappeared or diminished 

 considerably ; that if the clouds were high and the weather calm, 

 dew sometimes formed to no very inconsiderable extent ; that 

 dew often forms on shaded grass even several hours before sun- 

 set, and continues to form after sunrise ; that, if the weather be 

 favourable, more dew forms a little before, and, in shaded 

 places, a little after sunrise, than at any other time ; that on 

 substances elevated a few feet above the ground it forms much 

 later in the evening, but continues to form as long after the 

 rising of the sun as upon the ground ; that dew is more abun- 

 dant shortly after rain than during a long trnct of dry weather ; 

 that dew is always very copious on those clear and calm nights 

 which are followed by misty or foggy mornings, and also on 

 clear mornings after clou.ly nights, and generally after hot days ; 

 that more dew was formed between midnight and sunrise than 

 between sunset and midnight, owing doubtless "to the cold of 

 the atmosphere being greater in the latter than in the prior part 

 of the night ; " that whatever diminishes the view of the sky 

 diminishes the quantity of dew ; that a substance placed on a 

 raised board of some extent acquired more dew on a very still 

 night than a similar substance lying on grass ; that bright metals 

 attract dew much less powerfully than other bodies, that a metal 

 which has been purposely moistened will often become dry 



NO. 1209. VOL. 47] 



though similarly exposed with bodies which are attracting dew, 

 and that wool laid upon a metal acquires much less dew than an 

 equal quantity laid upon grass in the immediate vicinity ; that a 

 metal plate on grass always became moist on the lower side 

 during the night, though the upper side was often very dry, but 

 that if the plate was elevated several feet in the air, the condition 

 of both sides was always the same, whether dry or ujoist ; that 

 wool on a raised board was commonly colder than on the grass 

 on very still nights, and that the leeward side of the board was 

 colder than the windward ; that bare gravel and garden mould 

 were very much warmer after sunset than neighbouring grass ; 

 that on dewy nights the temperature of the earth half an inch or 

 an inch beneath its surface was much warmer than the grass upon 

 it, and than the air ; that metal covering grass was only slightly 

 colder than the grass covered, and this again colder than the 

 earth ; that metal thus exposed was w armer than air 4 feet above 

 it, and much warmer than neighbouring grass ; that the variety 

 in the quantities of dew, formed upon bodies of the same kind 

 in different situations, was occasioned by the diversity of tem- 

 perature existing among them ; and that on nights favourable to 

 the production of dew, only a very small part of what occurs is 

 owing to vapour rising from the earth. 



The last of these conclusions Wells supported by the obser- 

 vation that the dew on the grass increased considerably about 

 sunset, the same time at wh ch dew began to show itself on the 

 raised board, and by the reflection that, " though bodies situated 

 on the ground after they have been made sufficiently cold by 

 radiation to condense the vapour of the atmosphere will be able 

 to retain the moisture whiih they acquire by condensing the 

 vapour of the earth ; yet, before this happens, the rising vapour 

 must have been greatly diminished by the surface of the giound 

 having become much colder." He adduced the fact that 

 substances on the raised board attracted rather more dew 

 throughout the night than substances lying on the grass. He 

 admitted that all the dew on calm, cloudy nights might be at- 

 tributed to condensation of the earth's vapour, since on such 

 nights the raided board was dry. 



But if the grass was moist on these calm, cloudy nights, and 

 the moisture were owing to earth-vapour, it is only reasonable 

 to infer that a very much larger quantity was owing to earth- 

 vapour on clear nights when radiation was comparatively free. 

 Moreover, the fact that substances on the raised board became 

 wetter than substances on the grass may be attributed to the 

 non-conducting wood intercepting the warmth radiated from the 

 ground, and thus allowing a substance on the 'Upper surface of 

 the board to become colder than a substance on the grass. And 

 with regard to the " rising vapour " being greatly diminished by 

 the surface of the ground having become colder, it does not 

 appear that such diminution actually occurs, owing possibly to 

 the influence of the high temperature of the preceding day reach- 

 ing the moist earth at a little depth below the surface about the 

 same time. I have found the deposition of earth-vapour to pro- 

 ceed at a rapid rate after sunrise over grass. 



Wells explains with much ingenuity the reason why leaves of 

 trees often remain dry throughout the night, while those of grass 

 are covered with dew. But he does not, 1 think, attach 

 sufficient weight to the fact which he mentions among others, 

 that the air near the ground is near one of its sources of moisture, 

 while the tops of trees are removed from that source. The air 

 is both damper and colder near the ground ; a stratum of cooled 

 air rests upon warm earth emitting vapour. The tops of trees 

 are pervaded by air which is drier and warmer, and the leaves 

 do not allow air to rest long enough on their cooled surfaces to 

 part with sufficient heat in order that condensation may ensue. 



I have found that when the air is clear and nut humid, radia- 

 tion into space is often not sufficient to cause visible dew or frost 

 except in sheltered calm places, and in the same condition of 

 air deposition takes place more on broad surfaces than on thin 

 shoots, threads, and points, and more on the faces than on the 

 edges of leaves. It appears necessary that a certain stability of 

 temperature below that of the air, and a certain protection from 

 re-absorption by the drier portions of air which pass over, should 

 be attained in order that dew and frost may accumulate. When, 

 on the other hand, the air is very moist, with a tendency to 

 mist or fog, a very large condensation takes place on exposed 

 objects, and especially on those which are at some height above 

 the ground, such as the branches and twigs of trees. Points, 

 thorns, spiders' webs, and other thin filaments are then heavily 

 bedewed. Mist or fog often follows. 



When some mist has formed on such a night, there is a heavy 



