22 MR JOHN AITKEN ON DEW. 



be made during the whole year, to determine whether this evaporation is 

 constantly going on or not in fair weather, and to determine its amount 

 under different conditions. The varieties of soils, of humidities, and exposures 

 are so great that an enormous number of experiments would require to be 

 made to determine with any degree of accuracy the amount of evaporation 

 that takes place from any large tract of land. 



The temperatures of the soil and of the air during these experiments were 

 not high, but we must remember they were taken in October. In summer we 

 have to deal with much higher temperatures and greater vapour tensions, and 

 therefore the possibilities of heavier dews. On the 18th August I find the 

 temperature \ inch under the surface of the soil at 4 p.m. was 82°, at 3 inches 

 underneath it was 72°, the temperature of the air being 66°. At 9 p.m., at 3 

 inches deep, the temperature was 60° under grass and under bare soil. The 

 temperature on the grass was 45°, while a thermometer placed on bare soil 

 was 52°. Next morning the temperature at 3 inches under grass was 56°, 

 and at the same depth under soil 52°. The soil at 3 inches down had thus lost 

 20 degrees during the night, and that nearer the surface would have lost a 

 good deal more. Much of this loss would be spent in evaporating moisture. 

 On this occasion it will be noticed that at night the difference between the 

 temperature on the surface of the grass and on the bare soil was as much as 

 7 degrees, and this easily explains why the ground kept dry while the grass 

 got wet. 



So far as my limited observations go, evaporation is constantly going on 

 from soil under grass, but on a few occasions it was doubtful whether the reverse 

 process had not taken place, and vapour got condensed on the surface of bare 

 soil. On one or two occasions in autumn, I observed soil which had been dry 

 the previous day to be damp in the morning. The soil had evidently received 

 an increase of moisture. But the question still remains, Whence this 

 moisture ? Came it from the air, or from the soil underneath ? The latter 

 seems the more probable source, as the higher temperature below would 

 determine a movement of the moisture upwards by the vapour diffusing ; and 

 the surface soil being cold, the vapour would be trapped by it before it escaped 

 into the air, in the same way as it is trapped by grass on grass land. 



•During summer it is difficult to trace the vapour condensed on the surface 

 of the soil to its source, and to say definitely whether it came from the air or 

 from the ground underneath. But on the morning of the 12th October I had 

 an interesting opportunity of studying this question. During the night the 

 radiation had been very powerful, the surface of the soil was greatly cooled, 

 and a thin crust of frozen earth formed. After the sun had thawed the sur- 

 face it was very wet. An examination of the soil before the sun had acted on 

 it, showed that the vapour condensed near its surface had come from under- 



