88 



Observations on /evaporation. 



1844.] 



The average evaporation on the tank is therefore £th less 

 than that on the land, or the evaporation on the land is |th 

 more than that on the tank, during the hottest months of 

 the year. 



The gradual increase however of the proportional evapo- 

 ration on the tank which is so plain, is a circumstance which 

 it is difficult to assign any certain reason for. 



In order to make the observations as complete as circum- 

 stances would allow, the temperature of the water in both 

 evaporators, the temperature of the water at the surface and 

 bottom of the tank, as well as the temperature of a Thermo- 

 meter freely exposed to the sun, were observed at frequent 

 intervals during the day. But neither an examination of 

 these results, nor the general hygrometric state of the air, 

 nor even the force of the wind or state of the weather satisfac- 

 torily account for the gradual increase, which I have above 

 alluded to. 



I cannot help thinking that it must in a great measure be 

 owing to the general diminution of the depth of water in the 

 tank, the surface of which from the beginning of April to 

 the 20th of August fell 6 feet 3 inches. 



If this should be found upon further trial to be the case, it 

 will render still more apparent the necessity of attending to 

 depth in forming such reservoirs, the amount of evaporation 

 increasing not only in proportion to the surface exposed, but 

 inversely as the depth of water. 



What an important item evaporation is in the consumption 

 of water, is very apparent from these observations. 



During these 5 months the level of the water sunk 75 

 inches, notwithstanding falls of rain amounting to 8 inches, 

 so that the total fall was 83 inches, but the amount of 

 evaporation measured during this time on the tank was 53 

 inches so that only f ths of the quantity which disappeared 

 could have been available for the purpose of irrigation, f ths 

 being carried off by evaporation. 



