WATER WITHDEAWN FOR lERIGATION". 449 



Italy the summer rain is more abxmdant than it was before irri- 

 gation was practiced — for we know nothing of the meteorological 

 conditions of that country at so remote a period — the fact that 

 there is a very considerable precij^itation in the summer months 

 in Lombardy is a strong argument in favor of such increase. In 

 the otherwise similar climate of Rumeha and of much of Asia 

 Minor, irrigation is indeed practiced, but in a relatively small 

 proportion. In those provinces there is little or no summer rain. 

 Is it not highly probable that the difference between Italy and 

 Turkey in this respect is to be ascribed, in part at least, to exten- 

 sive irrigation in the former country, and the want of it in the 

 latter ? It is true that, in its accessible strata, the atmosphere of 

 Lombardy is extremely dry during the period of irrigation, but 

 it receives an immense quantity of moistm'e by the evaporation 

 from the watered soil, and the rapidity with which the aqueous 

 vapor is carried up to higher regions — where, if not driven else- 

 where by the wind, it would be condensed by the cold into drops 

 ■of rain or at least visible clouds — is the reason why it is so httle 

 perceptible in the air near the ground.* 



But the question of an influence on temperature rests on a 

 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 evaporation. 

 Hence, though the summer temperature of Lombardy is high, 

 we are warranted in affirming that it must have been still higlier 

 before the introduction of irrigation, and would again become so 

 if that practice were discontihued.f 



The quantity of water artificially withdrawn from running 

 streams for the purpose of irrigation is such as very sensibly to 

 afifect their volume, and it is, therefore, an important element in 

 the geography of rivers. Brooks of no trifling current are often 



* Is 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 ? 



f I do not know that observations have been made on the thermometric in- 

 fluence 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. 



