142 The Realm of Nature CHAP. 



heated land throws off as much of its air- covering as 

 possible, piling it upon the colder sea which eagerly 

 draws it down. While the land is throwing off the air 

 above, which descends upon the sea, the sea commences to 

 return it to the land along the surface more slowly than it 

 receives it at first, more rapidly afterwards. When the land 

 has drawn down on itself from above a greater supply of 

 air at the opposite season the sea gradually draws it off 

 along the surface. There is thus a constant effort to restore 

 the equilibrium of atmospheric covering between land and 

 sea, disturbed by the rapid radiation of the land. The 

 prevailing winds of the year, disregarding minor seasonal 

 changes, are shown in Plate VII. 



200. Rainfall and Evaporation. A continual circula- 

 tion of water takes place between the hydrosphere and 

 atmosphere. Sea winds blow water -vapour against the 

 land and ascending currents carry it into the upper atmo- 

 sphere, where it condenses and returns either directly as 

 rain or through springs and rivers to the sea. The amount of 

 evaporation at sea, and of rain falling on land depend mainly 

 on temperature and winds. Dr. John Murray has calculated 

 that nearly 130 million million tons of water, or about -^ of 

 the whole mass of the atmosphere, are transferred from the 

 sea surface to the land, and find their way back again in 

 rivers every year. More than half of the rain falls between 

 the tropics, and probably not more than of it reaches 

 ground as snow beyond the polar circles. 1 The average 

 rainfall of the globe is about thirty -three inches. A 

 calculation has been made that one quarter of the land 

 surface has a rainfall less than one foot in a year, one 

 quarter has a rainfall between one and two feet, one quarter, 

 of which the British Islands form part, has a rainfall of 

 between two and four feet, and over the remaining quarter 

 the rainfall exceeds four feet in a year. In all regions not 

 reached by sea winds the rainfall is very slight, and 

 evaporation preponderates, a nearly rainless area containing 

 dwindling salt lakes occupying part of the interior of each 

 continent ( 356). 



201. Distribution of Rainfall. Plate VIII., the data 



