MR JOHN AJTKEN ON DEW. 51 



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An inverted tray was placed over bare soil which was so dry and powdery 

 that it rose in dust when the edge of the tray was pressed into it. The tray 

 was not placed on the ground till after 9 p.m. — that is, after the soil had lost a 

 good deal of heat and moisture — yet in two hours the inside was wet, and in 

 the morning it was covered with drops. While this experiment was going on, 

 another tray was placed on the lawn at a place where the grass was burnt quite 

 brown. The inside of this tray was also wetted, and to about the same amount 

 as the one over the bare soil. It may be noted here, that the positions for 

 making these tests were selected on account of their extreme dryness. The 

 bare soil was light and open, while the lawn was extremely dry, having been 

 laid down a few years ago with every precaution to ensure this condition ; the 

 under soil is dry and sandy, it was moreover well drained, the upper soil was 

 removed, and a good depth of ashes put in its place, then a layer of sand, on 

 which was put the turf with its inch or two of soil. A drier position could 

 scarcely have been selected for the trial, and yet the result showed that in our 

 climate bare soil and grass land, even when very dry, continue to give off vapour 

 during dewy nights. 



A few experiments were also made on this subject in March last at Hyeres, 

 in the south of France. In order that the test might be as severe as possible, 

 I selected a spot where the soil seemed driest and most exposed to the sun and 

 the wind; and as the soil was stony and lying on the hill-side, it could 

 have no supply of water from below. An inverted tray placed over this arid 

 ground collected a surprisingly great amount of moisture. The following are 

 the notes of one of the tests made at Hyeres on the 27th March last :— At 5.15 

 p.m. the sun had ceased to shine on the place selected for the experiment ; at 

 that hour the temperature of the soil was 73° F. at 3 inches below the surface, 

 and 59° at 12 inches. At 5.45 the tray was put on the ground. Examined at 

 7.15 p.m. the outside was dry, but inside was quite wet, and by 10 p.m. the drops 

 were so large they ran down the inside when the tray was placed vertically. 

 This great amount of wetting was probably due, not only to the ground being 

 highly heated during the day, but also to a considerable fall in temperature at 

 night, by the cooling produced under the clear skies of that climate. 



I much regret I have been unable to get any trustworthy information 

 regarding the movement of the vapour near the surface of the ground in barren 

 and desert countries. I have, however, received some information from 

 travellers who have been in Australia and parts of South Africa, where rain 

 does not fall for months at a time, and it goes to prove that even in these dry 

 countries vapour rises from the ground at night, as they often found the under 

 side of their waterproof bedding placed on the ground to be wet after camping 

 out at night. One would scarcely have expected much moisture to be collected 

 in this way on account of the warmth of the sleeper's body keeping up 



