464 WATER WITHDRAWN FOR IRRIGATION. 
moisture by the evaporation from the watered soil, and the ra- 
pidity with which the aqueous vapor is carried up to higher 
regions—where, if not driven elsewhere by the wind, it would 
be condensed by the cold into drops of rain or at least visible 
clonds—is the reason why it is so little perceptible in the air 
near the ground.* 
But the question of an influence on temperature rests on @ 
different ground; for though the condensation of vapor may 
not take place within days of time and degrees of distance from 
the hour and the place where it was exhaled from the surface, 
a local refrigeration must necessarily accompany a local evapo- 
ration. Hence, though the summer temperature of Lombardy 
is high, we are warranted in affirming that it must have been 
still higher before the introduction of irrigation, and would 
again become so if that practice were discontinued.t 
The quantity of water artificially withdrawn from running 
streams for the purpose of irrigation is such as very sensibly 
to affect their volume, and it is, therefore, an important element 
in the geography of rivers. Brooks of no trifling current are 
often wholly diverted from their natural channels to supply the 
canals, and their entire mass of water is completely absorbed 
or evaporated, so that only such proportion as is transmitted by 
infiltration reaches the river they originally fed. Ivrigation, 
therefore, diminishes great rivers in warm countries by cutting 
off their sources of supply as well as by direct abstraction of 
water from their main channels. We have just seen that the 
system of irrigation in Lombardy deprives the Po of a quan- 
tity of water equal to the total delivery of the Seine at ordinary 
flood, or, in other words, of the equivalent of a tributary navi- 
gable for hundreds of miles by vessels of considerable burden, 
* Ts not the mottled appearance of the upper atmosphere in Italy, which I 
have already noticed, perhaps due in part to the condensation of the aqueous 
vapor exhaled by watered ground ? 
+ I do not know that observations have been made on the thermometric 
influence of irrigation, but I have often noticed that, on the irrigated plains 
of Piedmont ten miles south of Turin, the morning temperature in summer 
was several degrees below that marked at the Observatory in the city. 
