MR JOHN AITKEN ON DEW. 21 



At 10.40 p.m. 



Temperature of air, Dry bulb, 38° Wet bulb, 36° at 4' 0" 



n ii n 33^ ii 33° near ground. 



Grass. Soil. 



Temperature on surface, 31° 34° 



n of soil, 3 inches below surface, 44°-2 43° 



12 ii „ 46° 46° 



Grass exposed, 



lost 



9 grains, or 



- 583 grammes. 



H under tray, 



ii 



8 „ 



0-518 



ii 





Soil exposed, 



ii 



16i - 



1-069 



H 





ii under tray, 



ii 

 Next 



9 ,i 



MORNING AT 9 A.M. 



0-583 



Grass. 



n 



Soil. 



Temperature on surface, 





39°-5 





39°-5 



ii of soil, 



3 inches under surface, 42° # 5 





40°-5 



ii ii 



12 



ii n 



45°-5 





45°-5 



Grass exposed, 



lost 



19 grains, or 



1"231 grammes. 



ii under tray, 



It 



13 n 



0-842 



ii 





Soil exposed, 



II 



30 .. 



1-944 



H 





M under tray, 



II 



18 „ 



1-166 



ti 





These figures cannot be supposed to represent anything definite, they 

 only indicate a condition of matters which has not been previously observed. 

 They show that evaporation in our climate is going on night as well as day 

 during dry weather, but the extent to which it takes place cannot be gathered 

 from these observations, as they are far too few for the purpose — too few alike 

 with regard to seasons, humidities, and exposures ; nor can the proportionate 

 amount of evaporation from bare soil and from grass-land be arrived at from 

 the weights given. These readings can only be considered true for the place 

 and moisture at the time of the year when the experiments were made. For 

 instance, the inverted trays over soil in my early experiments always indicated 

 a larger evaporation from soil than from grass, while the later ones did not. 

 But the early experiments were made over soil freely exposed to sunshine 

 during the whole day, while the later ones were made at a place less freely 

 exposed, on account of the situation where the first experiments were made 

 being too far from the place of weighing. It is evident the amount of sunshine 

 will be an important factor in this nightly evaporation, as it will greatly 

 determine the amount of heat stored up during the day, and available for 

 evaporation during the night. 



I extremely regret the season was so far advanced before these experiments 

 were begun, as most of the weather suitable for the purpose was past. I have, 

 however, endeavoured to check my results as well as possible. Still I feel that 

 what has been done is only preliminary. Similar experiments would require to 



