PROPORTIONS OF LAND AND WATER. 473 



and North Temperate Zones are not very unequally divided between land and water. 

 The ocean covers four-fifths of the Tropical Zone, and rolls in an almost unbroken mass 

 over the southern temperate and Antarctic regions. This great preponderance of 

 water over land between the tropics is one of the most important facts in physical 

 geography. Were the proportions reversed, sterility would be the rule all over the 

 globe, for without water there can be no vegetable or animal life. All the water that 

 wells up in fountains, or flows in brooks and rivers, comes from the ocean, whence it 

 is raised by evaporation and borne along the viewless channels of the air to be pre- 

 cipitated in the form of rain and snow, sometimes thousands of miles from the spot 

 where it commenced its aerial journey. It is computed that nearly 200,000 cubic 

 miles of water are annually raised from the ocean in the form of vapor. Three- 

 quarters of this is raised within the tropics, and a great part falls beyond them. If 

 the extent of the tropical ocean were diminished by half, there is hardly a part of the 

 Temperate Zone which would not be parched by perpetual drought, and hardly a river 

 whose bed would not become a dry ravine. The hidden springs of the Amazon, 

 Mississippi, and the Danube lie in the Pacific. The water which fills the great 

 lakes of North America, and thundering down the cataract of Niagara finds its way 

 through the St. Lawrence River into the ocean almost on the verge of the polar 

 'world, only a few weeks before, perhaps, laved the coral reefs of the tropical seas. 



Moreover, if any considerable part of the tropical ocean were converted into land, 

 the heat of the Torrid Zone would become so greatly enhanced that no animal life, such 

 as now exists, could endure it ; and as the vegetation of a climate is adapted to the 

 prevailing temperature, the trees and plants which now flourish would become extinct. 

 Water in being converted into a gaseous form by the process of evaporation absorbs 

 heat from surrounding objects, or, as we say, produces cold. Thus the burning rays 

 of a vertical sun pouring down upon the ocean in a measure quench themselves. The 

 same rays which falling upon the ocean never raise the water beyond a grateful 

 temperature, falling upon the land produce an intolerable heat. To step on a sum- 

 mer's day from the cool water upon the sandy beach is like treading upon a plate of 

 heated metal. 



The conformation of the land within the tropics likewise goes far to counterbalance 

 or mitigate the excessive heat of a vertical sun. The most casual glance over a map 

 shows that the land here is mostly insular, laved on all sides by the surrounding 

 waters, or stretches in a narrow length between two oceans, thus multiplying the 

 surface over which the sea is enabled to exert its cooling influence. 



The great extent of the tropical seas is also the primary cause of those mighty ocean 

 currents which sweep from the equatorial to the polar regions. Cool as is the water of 

 the tropics when compared with the land, it is yet warm when compared with the other 

 parts of the ocean. The water thus heated becomes specifically lighter than that 

 of colder regions, is lifted up, and, in obedience to the law' of gravitation, runs off in 

 both directions toward the poles. There having become cooled, the salt waters are 

 heavier than the comparatively fresh ones of the polar regions, and, sinking beneath 

 them, return in an under current to their starting-place. 



This great equatorial current, or rather series of currents, is the marvel of physical 

 geography. Let us follow that of the Atlantic in its long career. Starting on the 

 line of the equator, it flows north-westward along the coast of South America, enters the 



