MR JOHN AITKEN ON DEW. 17 



The experiments were generally stopped at night. It would be of no use to 

 let them go on till morning, unless one were in attendance at sunrise; for the 

 early morning heat radiated from the sun and sky would cause an increased 

 evaporation, and make the loss appear too great. On one occasion, however, 

 when the morning was dull, weighings were made, and the soil was then found 

 to have lost weight during the late night and morning. 



The following simple observation is sufficient to convince us that, under the 

 ordinary conditions of our climate, vapour is almost constantly escaping night 

 and day from soil under grass. Go out any night, but it is best when terrestrial 

 radiation is strong, place one thermometer on the grass, and push another under 

 its surface, among the steins, but it need not be into the soil, and note the differ- 

 ance in temperature. As an example, I found, at 10.45 p.m. on the 10th October, 

 this difference to be as much as 185 degrees. The thermometer on the surface 

 of the grass was 24°, while the other, only about 1^ inches underneath it, and 

 not in the soil, was as high as 42° 5, the temperature of the air at the time 

 being 32° o. Of course, this difference varies, and is not always so great as on 

 this occasion, when the sky was clear and the air still. An experiment of this 

 kind causes us to doubt the value of the radiation observations made by com- 

 paring the readings of a thermometer placed on the grass with the temperature 

 of the air in the screen ; because the temperature of the thermometer on the 

 grass varies greatly according to its position. If its bulb is supported near the 

 tips of the stems, the temperature is much lower than when it is allowed to 

 press the grass close to the ground, because in the latter position it receives a 

 good deal of heat from the earth. 



It might be objected that these experiments having been made late in the 

 year,, and when the soil was damp, they do not prove that evaporation would 

 take place in summer when the soil was dry. Other considerations, however, 

 lead us to suppose that this nightly evaporation does go on even after a con- 

 tinuance of dry weather, though I have no direct experiments to prove it, other 

 than those made with the inverted trays. But I find that soil, after it has been 

 kept for some time in a house, and when it looks dry and incapable of support- 

 ing vegetation, still gives off vapour, and saturates the air over it. This was 

 shown by placing over some dry-looking soil a glass receiver, in which was 

 hung the hair hygrometer. The instrument soon showed an incease of humidity 

 inside the receiver, and after a time indicated saturation. To check the read- 

 ing of the hygrometer, it was quickly removed and placed in saturated air, when 

 it was not found to change its reading. 



Now as soil, even when it appears dry, tends to give off vapour, and saturate 

 the air in contact with it, it is evident that under most conditions of our 

 climate the vapour tension at the surface of the ground, amongst the stems of 

 the grass, must, owing to the higher temperature, be very much greater than 



VOL. XXXIII. PART I. C 



