226 LIGHT SCIENCE FOR LEISURE HOURS. 



when there is a failure in the supply of rain, or when 

 that supply comes out of its due season. When we 

 consider rain in connection with the causes to which 

 it is due, we begin to recognise the enormous amount 

 of power of which the ordinary rainfall of a country is 

 the representative ; and we can well understand how it 

 is that ' the clouds drop fatness on the earth.' 



The sun's heat is, of course, the main agent we 

 may almost say the only agent in supplying the rain- 

 fall of a country. The process of evaporation carried 

 on over large portions of the ocean's surface is con- 

 tinually storing up enormous masses of water in the 

 form of invisible aqueous vapour, ready to be trans- 

 formed into cloud, then wafted for hundreds of miles 

 across seas and continents, to be finally precipitated 

 over this or that country, according to the conditions 

 which determine the downfall of rain. These processes 

 do not appear, at first sight, indicative of any very 

 great expenditure of force, yet in reality the force- 

 equivalent of the rain-supply of England alone for a 

 single year is something positively startling. It has 

 been calculated that the amount of heat required to 

 evaporate a quantity of water which would cover an area 

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

 to the heat which would be produced by the combustion 

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

 which this consumption of heat would be the equiva- 

 lent corresponds to that which would be required to 

 raise a weight of upwards of one thousand millions of 

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



