52 MR JOHN AITKEN ON DEW. 



the temperature of the condensing surface. It therefore seems probable 

 that the moisture will collect under those parts of the waterproof which are 

 beyond the influence of the sleeper's body. 



While the experiments above referred to on bare soil and grass land were 

 being made on ground exceptionally dry in July, those with slates placed on 

 the road were repeated. As in the experiments described in the first part of 

 this paper, the slates placed on the hard dry part of the road and on the 

 gravel got quite wet on their under sides at night, thus showing that, even 

 under the exceptionally dry conditions existing at the time, vapour was still 

 rising at night from the hard and arid roads. 



In connection with the action of stones lying near the surface of the ground, 

 we may here refer to a result observed by agriculturists, on which it seems to 

 throw some light. It has been remarked that the removal of small stones from 

 fields where the soil is light and open, often has a prejudicial effect on the 

 crops. -It must be admitted that accurate information on this point is not 

 easily obtained, but the impression in some parts of the country certainly is, 

 that the removal of small stones from fields is not to be recommended. Now 

 the removal of stones may act prejudicially in different ways. In some cases 

 the disintegration of the stones may add to the richness of the soil ; their 

 removal may therefore in some cases decrease the natural fertility of the field. 

 No doubt the removal of the stones will permit of a more rapid evaporation from 

 the soil during the day, but it does also seem probable that the peculiar action 

 of stones, in trapping the moisture before it comes to the surface at night, will 

 have a beneficial effect on light and dry soils. Part of the moisture is trapped 

 by them before it can escape from the surface at night ; and when the sun 

 rises, the stones becoming warmer than the soil under them, the moisture 

 leaves the stones and condenses in the soil underneath. In this way stones 

 would seem to have a sort of conserving action on the moisture, and tend to 

 check the prejudicial effects of continued dry weather. 



In confirmation of this conserving action of bodies lying on the surface of 

 the ground and improving its fertility, I would refer to a letter in Nature, 

 vol. xxxiii. p. 583, by Lieut. -Col. A. T. Fraser. As this letter is so 

 interesting, and bears directly on our subject, I may be allowed to quote 

 from it here. At the beginning of his letter he says : — " Having had occasion 

 to lay out a large quantity of iron hoes and picks, without handles, on the hard 

 ground of an open inclosure in one of the driest districts of India (Bellary), 

 where, in fact, these implements had been collected in the face of a scarcity, it 

 was found, after they had lain a couple of months, that a thick, weedy, but 

 luxuriant vegetation had sprung up, enough, though there was no rain, to 

 almost hide the tools." 



Lieut.-Col. Fraser also says he had previously noticed in the tropics a 



