250 LIGHT SCIENCE FOR LEISURE HOURS. 



been calculated that the amount of heat required to 

 evaporate a quantity of water which would cover an 

 area of 100 miles to a depth of one inch would be equal 

 to the heat which would be produced by the combus- 

 tion of half a million tons of coals. The amount of 

 force of which this consumption of heat would be the 

 equivalent, corresponds to that which would be required 

 to raise a weight of upward of one thousand millions 

 of tons to a height of one mile. Now, when we re- 

 member that the area of Great Britain and Ireland is 

 about 120,000 square miles, and that the annual rain- 

 fall averages about 25 inches, we see that the force- 

 equivalent of the rainfall is enormous. All the coal 

 which could be raised from our English coal-mines in 

 thousands of years would not give out heat enough to 

 produce England's rain-supply for a single year. When 

 to this consideration we add the circumstance that the 

 force of rain produces bad as well as good effects the 

 former when the rain falls at undue seasons or in an 

 irregular manner, the latter only when the rainfall is 

 distributed in the usual manner among the seasons 

 we see that an important loss accrues to a country in 

 such exceptional years as the present. 



There are few subjects more interesting than those 

 depending on the correlation of physical forces ; and 

 we may add that there are few the study of which bears 

 more largely on questions of agricultural and commer- 

 cial economy. It is only of late years that the silent 



