On Evaporation. 
in the ensuing summer there fell 11-61 inches of rain; and in the 
month of September following, the fall of rain, for that month, 
amounted to 3-32 inches, while the rain for September of last 
year amounted to about 0-60 of an inch more than the precedino-. 
In connection with this it usually happens, that the amount of 
evaporation is also greater during a dry season, as the atmosphere 
rarely becomes sufficiently saturated to produce rain, there is a 
constant increased exhaustion with a diminished supply. There- 
fore, assuming the year 1846, from observation, we may say that 
one and a half inches of rain in May, three inches in June, four 
in July and August each, affords a standard year of fruitfulness 
and productiveness. It naturally follows that evaporation bein<y 
nearly constant in its operations, those parts which are most 
exposed are in the greatest degree affected by it. A field covered 
WMth thick verdure, will be less exposed to its influence, than one 
that has no crop upon it; so, too, a pasture field, which is con- 
stantly nipt by horses and cattle, will in a drought suffer much 
from its want of covering, as well as loss of moisture from the 
vessels of the plants being cut asunder in the operation of depas- 
turing. Winds also have a powerful influence upon evaporation; 
hence, situations much exposed to them will be affected in pro- 
portion to their prevalence. Evaporation from a water surface 
being more rapid than from any other object, is also, of course 
controled by the degree of heat applied to that surface. Soils 
too, will be affected differently by heat; a silicious or sandy soil, 
from the little cohesion in its particles, will give out its moisture 
more quickly, w^hile an argillaceous or clayey one, from its adhe- 
sive nature, will part less readily with it; hence, as either of these 
substances prevailed, the evaporation would be modified, so also 
if either contained a quantity of vegetable matter, more or less it 
would have its influence. A soil that forms a hard crust upon its 
surface, would be unfavorable to evaporation, by preventing the 
escape of vapor and transmission of heat, which is so important 
an agent in the process of evaporation. 
In some experiments which I made in the month of July, 1846 
I found that the water in a cylinder stood at 100^ exposed to the 
sun, which raised the thermometer to 115^ of Fahrenheit. Upon 
applying the thermometer to the surface of the ground, which 
was of an argillaceous character, and devoid of vegetation, it sunk 
to 90^, while six inches below the surface it sunk still further to 
78 \ which was the same as the temperature of the air at the time 
of the experiment. In the following table is contained the tem- 
perature of Lake Seneca, also of the air, and the amount of 
evaporation in hundredths of inches, during the last two weeks of 
July, of the above mentioned year: 
