EVAPORATION. 



135 



Altogetlier, it will have to be admitted that the factor of dissipation 

 represented in the evaporation from the gronnd is eon.siderably rednced 

 by the forest-cover; and since the rate of evaporation in onr Western 

 Territories is probably the greatest element in the dissipation of mois- 

 tnre, the greatest attention to checking it will be necessary in the hus- 

 banding of water supplies. This check to evaporation refers not only 

 to the preservation of the water supply where it falls, but also in the 

 natural and artificial channels through which it may be conducted or 

 in the reservoirs where it may be stored. 



The surface exposed determines the amount of evaporation from 

 water-courses and reservoirs; but if the amount evaporated is related 

 to the available volume of water, it wOl appear that the smaller and 

 slower run loses proportionately more than the larger, which thus ex- 

 hibits the value and protective (character of accumulation. 



Take a brook G feet in width and only a foot in depth ; this for a 

 length of 30 feet would contain 180 cubic feet of water. If from this 

 surface only one-tenth of 1 inch evaporates, the amount eva]>orated is 

 eqnal to 1.5 cubic feet or jl^ of the entire snpply. On the other hand 

 one-tenth of 1 inch evaporation from a river 00 feet broad and 12 feet 

 deep for a length of 30 feet, containing therefore 21,000 cubic feet of 

 water, would bring the loss to 15 cubic feet or only jtr o of the avail- 

 able supplies; the loss, in proportion to the supply, being twelve times 

 greater in the former case. 



