266 EVAPORATION. 



moves towards the earth. But there is reason to suppose, 

 that discharges are sometimes made from the earth to the 

 clouds, as well as from the clouds to the earth. It is not 

 difficult to measure the distance of thunder-clouds from 

 the earth. Sound moves at the rate of 1142 feet in a 

 second ; light at the rate of about 200,000 miles in a sec- 

 ond. The time in which light traverses so small a space, 

 as that between a thunder-cloud and any place, from 

 which the thunder can be heard, is so short that it need 

 not be estimated. If then we multiply the number of 

 seconds between the flash and the thunder by 1 142, we 

 have the distance of the cloud in feet. Hence, when a 

 very short time elapses between the flash and the thunder, 

 the discharge is very near. There is a peculiar sublimity 

 attending thunder-storms in mountainous regions. The 

 traveller among the Andes frequently hears the thunder 

 roll, and sees the lightning flash from the clouds that 

 gather around the hills far beneath him, while around his 

 path, and on the heights above him, the sun is shining 

 with unclouded splendor. 



As the quantity of water in the clouds is increased by 

 new accessions of vapor, they at length become too heavy 

 to be supported in the air, and begin to sink. Often 

 also the air below them grows lighter, and consequently 

 less able to support them. They will then descend with- 

 out any increase of weight. On this principle, the falling 

 of smoke indicates rain. It seems a very fair conclusion 

 that if the air is not sufficiently heavy to carry smoke up, 

 it will soon let the water which it contains, come down. 

 The barometer usually sinks before rain, showing that 

 one great cause of storms is a diminution of the weight 

 of the atmosphere. As the vapor descends, it will meet 

 with other portions of vapor, and by degrees turn into 

 drops of rain. To exhibit the process in a clearer light, 

 let us suppose a few particles of vapor suspended high in 

 the air, and strata of vapor arranged horizontally below. 

 As each particle of the higher vapor passes through the 

 strata below, a portion of the water in those strata, will 

 unite with it : thus a drop will be formed. As the air 



