IRRIGATION IN EUROPE. 443 



plj of water is unlimited, it is allowed, after serving its purpose 

 ■on one field, to run into drains, canals or rivers. But in most 

 regions Avliere irrigation is regularly employed, it is necessary to 

 economize the water ; after passing over or through one parcel 

 of ground, it is conducted to another ; no more is usually with- 

 drawn from the canals at any one point than is absorbed by the 

 soil it irrigates, or evaporated from it, and, consequently, it is not 

 restored to liquid circulation, except by infiltration or precipita- 

 tion. "We are safe, then, in saying that the humidity evaporated 

 from any artificially watered soil is increased by a quantity bear- 

 ing a large proportion to the whole amount distributed over it, 

 for most even of that which is absorbed by the earth is imme- 

 diately given out again either by vegetables or by evaporation ; 

 and the hygrometrical and thermometrical condition of the atmos- 

 phere in irrigated countries is modified proportionally to the 

 extent of the practice. 



It is not easy to ascertain precisely either the extent of surface 

 thus watered, or the amount of water supphed, in any given 

 country, because these quantities vary with the character of the 

 season ; but there are not many districts in Southern Europe 

 where the management of the arrangements for irrigation is not 

 one of the most important branches of agricultural labor. The 

 -eminent engineer Lombardini describes the system of irrigation 

 in Lombardy as, " every day in summer, diffusing over 550,000 

 hectares [1,375,000 acres] of land, 45,000,000 cubic metres 

 [nearly 60,000,000 cubic yards] of water, which is equal to the 

 entire volume of the Seine, at an ordinary flood, or a rise of three 

 metres above the hydrometer at the bridge of La Tournelle at 



air is still, as it usually is in very cold weather, the surface of the lake, as far 

 as the eye can reach, is dotted here and there with columns of fog, like the 

 smoke from a chimney in clear, cold weather, rising to the height of three 

 hundred feet or more, and then uniting into a long cloud. How far the heat 

 disengaged in the conversion of water into ice is an element in the production 

 of this effect, I will not undertake to say. Though not exactly to the purpose, 

 I may here notice the expansion of the ice in this lake with increased tempera- 

 ture. Two gentlemen well known to me, undertook, some years since, to 

 measure, on the ice of Lake Champlain, a base line for ascertaining trigono- 

 metrically the height of a mountain peak, but every change of atmospheric 

 temperature produced a change in the length of the line measured, and foi 

 this reason the experiment was abandoned. 



