the Laws of Evaporation and Absorption. 289 



ascensional current of vapour, which would facilitate the process 

 of evaporation ; but this can only take place during continued 

 sunshine : in other cases the temperature of the damp surface 

 (owing to the cooling effect of evaporation) is from 1 to 6 de- 

 grees below that of the surrounding air ; and under such circum- 

 stances, according to the results of these experiments, we can 

 have little or no ascensional current of vapour tending to pro- 

 mote the process of evaporation. 



The temperature and hygrometric state of the air being con- 

 stant, the process of evaporation is very much accelerated by 

 aerial currents and by direct sunshine. 



4. The rate of evaporation from a damp surface is veiy much 

 affected by the elevation at which the surface is placed above the 

 ground. 



About half an hour before sunset, with a clear sky and a calm 

 air, the two tin plates, with their damp surfaces somewhat below 

 saturation, were placed as follows : one about an inch above the 

 grass, the other at an elevation of three feet, both damp surfaces 

 being placed upwards. In the course of two hours the former 

 had gained 7 '5 grains of moisture by absorption, whilst the latter 

 had lost 5 grains of moisture by evaporation. In another expe- 

 riment, the temperature of the air being higher, the former had 

 lost 12'7 grains by evaporation, whilst the latter had lost 21*2 

 grains by evaporation. Here the damp cloth at the surface of 

 the ground was cooled down with the grass by radiation, whilst 

 the upper cloth was nearly maintained at the temperature of the 

 air at that elevation. 



5. The rate of evaporation is affected by the radiation of sur- 

 rounding bodies. 



The plates, as described, were placed in the shade at the height 

 of six inches above the ground, with their damp surfaces up- 

 wards, the sky being clear, but one plate had a screen placed 

 over it. In the course of two hours the latter had lost 8 grains 

 by evaporation, whereas the former had lost only 3*5 grains ; 

 that is, the evaporation from the surface with the screen was 

 about double that from the surface without the screen. On 

 another occasion the evaporation from the surface with the screen 

 over it was found to be 32*6 grains, whilst from the other surface 

 it was only 25 grains. 



Hastings, February 16, 1862. 



[To be continued.] 



