258 WRITINGS OF JOSEPH HENRY. [1855- 



which are naturally connected with the general subject of 

 the existence of vapor in the atmosphere. 



The humidity so constantly supplied to the air by evapora- 

 tion is returned to the surface of the earth principally in the 

 form of rain resulting from the union of the very minute 

 particles of water which constitute the mass of clouds. With- 

 out stopping to inquire into the cause of union in this place 

 we may remark that we think it probably due to the further 

 condensation of the vapor which first assumed the condition 

 of a cloud. Rain, it is true, has been observed to fall from 

 apparently a cloudless sky, but the occurrence is one of ex- 

 treme rarity, and it seems possible that it is brought from a 

 distance by wind at a high level. 



A knowledge of the quantity of rain which falls in differ- 

 ent portions of a country is important, not only with refer- 

 ence to agriculture, but also with reference to internal 

 navigation, as well as to the application of hydraulic power, 

 the occurrence of devastating floods, the water supply of 

 cities, and the sanitary condition of a district. 



Almost every portion of the earth on which rain falls i& 

 provided with natural drains that carry off the surplus 

 water (above that which evaporates) to the ocean whence it 

 came; and taking the earth as a whole the same amount of 

 water must be returned to the ocean as was taken from it by 

 evaporation. 



Nearly the whole surface of the earth is divided intc> 

 basins, each provided with a separate system of drainage. 

 The boundaries of these basins can readily be traced on the 

 map b}' drawing a line around between the heads of the 

 streams, the waters of which find the level of the ocean 

 through the channels of different rivers. Thus we have the 

 great primary basins of the Amazon, the Mississippi, and the 

 St. Lawrence, and the secondary basins of the Ohio, the Mis- 

 souri, and the Tennessee, giving the latter name to those 

 which pour their waters not into the ocean but into another 

 river. 



A knowledge of the amount of rain which falls on each of 

 the subordinate basins supplying a river like the Mississippi 



