/212 OANAL. 



the mean evaporation from the green ground 

 25.14, and from water 44.43. From this it ap- 

 pears that the evaporation from a surface of wate^j 

 is nearly twice as muclias from green ground ; and 

 also, that about eight or nine inches of rain are 

 left for the supply of springs and rivers. This 

 surplus of water must be drawn from the sea, and 

 must return to it again by rivers. It must be 

 obvious that these experiments and estimates are 

 by no means unerring. But it is supposed that 

 the mean annual evaporation over the whole sur- 

 face of the earth, is 35 inches for every square 

 inch, and that therefore 94,450 cubic miles of 

 water are annually evaporated over the whole 

 globe. 



Considering the climate of the country through 

 which the canal runs — the great heat of the sum- 

 mer— ^the protracted autumn — and the compari- 

 tively mild winter, it is not unreasonable to in- 

 crease the general calculation in its application 

 to that region five inches on green earth, and ten 

 inches on water. The whole annual evaporation 

 of the middle section of the western canal, reckon- 

 ing that each square inch evaporates in that time 

 45, will only amount to 9,212,444 hogsheads — 

 a loss certainly of no great consequence when wO 

 covisider the supply. 



