448 METEOEOLOGICAL EFFECTS OF lEEIGATIOlSr. 



The rice-grounds and the marcite of Lombardy are not included 

 in these estimates of the amount of water apphed.* 



The meteorological effect of irrigation on a large scale, which 

 would seem jpriTna facie most probable, would be an increase of 

 precipitation in the region watered.f Hitherto scientific observa- 

 tion has recorded no such increase, but in a question of so purely 

 local a character, we must ascribe very great importance to a con- 

 sideration which I have noticed elsewhere, but which has been 

 frequently overlooked by meteorologists, namely, that vapors ex- 

 haled in one district may very probably be condensed and pre- 

 cipitated in another very distant from their source. If then it 

 were proved that an extension of irrigated soil was not followed 

 by an increase of rainfall in the same territory, the probability 

 that the precipitation was augmented somewhere would not be in 

 the least diminished. 



But though we can not show that in the irrigated portions of 



rived from the Dora Baltea and the Sesia, both because it is warmer, and 

 because it transports a more abundant and a richer sediment than the latter 

 streams, which are fed by Alpine ice-fields and melting snows, and which flow, 

 for long distances, in channels ground smooth and bare by ancient glaciers 

 and not now contributing much vegetable mould or fine slime to their waters. 



* About one-seventh of the water which flows over the marcite is absorbed 

 by the soil of those meadows or evaporated from their surface, and conse- 

 quently six-sevenths of the supply remain for use on ground at lower levels. 



f On the pluviometric effect of irrigation, see LoirBARDiNi, Sulle Irion- 

 dazioni, etc. , pp. 72, 74 ; the same author, EBsai Hyclrologique sur le Nil, p. 32 ; 

 Messedaglia, Analisi dell' opera dl Champion, pp. 96, 97, note; and Baikd 

 Smith, Italian Irrigation, i., pp. 189, 190. 



In an article in Aus der Natiir, vol. 57, p. 443, it is stated that the rain on 

 the Isthmus of Suez has increased, since the opening of the canal has enlarged 

 the evaporable surface of the country ; but this can not be accepted as an 

 estaMished fact without further evidence. Nevertheless, although I am not 

 able to adduce exact pluviometrical measurements, it appears now (1882) to be 

 admitted that since the filling of the Bitter Lakes and other depressions on the 

 Isthmus of Suez, by the opening of the canal, there has been a marked in- 

 crease in the quantity of precipitation falling there, and that rain, which was 

 formerly a rare and exceptional phenomenon, now falls, at certain seasons, 

 with considerable regularity. The Bitter Lakes are computed by Lesseps to 

 contain 2,000,000,000 cubic metres of water, and the annual evaporation from 

 this surface is estimated at 200,000,000 cubic metres. It is also said that the 

 rainfall has diminished in the old bed and the environs of the Lake of Haar- 

 lem since its drainage. But I am unable to refer to any competent authority 

 in support of this assertion. 



