INTRODUCTION. 



floating ice, constantly moving in that direction. Another current, called the tropical current, 

 sets, within the tropics, from east to west ; so that vessels coming from Europe to America, 

 descend to the latitude of the Canaries, where they are carried rapidly westward, and in going 

 from America across the Pacific to Asia, a similar effect is observed. The Gulf Stream is 

 or\e of the most remarkable and best known of the oceanic currents. 



12. Whirlpools. When two opposite 

 currents of about equal force meet one 

 another, they sometimes, especially in 

 narrow channels, turn upon a centre and 

 assume a spiral form, giving rise to whirl- 

 pools. Sometimes the most violent of 

 these, when agitated by tides or winds, 

 become dangerous to navigators. 



13. Saltness of the Sea. The waters 

 of the sea hold in solution several salts, 

 among which are common salt or muriate 

 of soda, Epsom salt or sulphate of mag- 

 nesia, and Glauber's salt or sulphate of 

 soda. The saltness of the sea varies in 

 different places, but is generally less to- 

 wards the poles than near the tropics, and 



in mland seas or bays than in the open ocean. In some places springs of fresh water rise up 

 in the midst of the sea. The bitterness of sea-water is supposed to be owing to the decom- 

 posed animal matter which it contains. It is easier to perceive the great advantages arising 

 from the saltness of sea-water than to discover its origin. Without this saltness and without 

 the agitation in which its waves are constantly kept, the ocean would become tainted. It is 

 remarkable, that the saltness of the sea is less toward the poles than under the equator. 



14. Color of the Sea. The green and yellow shades of the ocean arise from marine plants. 

 The sparkling of the sea at night presents an imposing spectacle. A vessel, while ploughing 

 her way through the billows, often appears to mark out a furrow of fire ; bometimes every 

 stroke of the oar of a boat, emits a light either brilliant and dazzling, or tranquil and pearly. 

 This wonderful phenomenon is supposed to be produced, at least in part, by multitudes of small 

 phosphorescent animals that live in the water. 



15. Temperature oj tlie Ocean. The temperature of the sea changes much less suddenly 

 than that of the atmosphere, and it is by no means subject to such extremes of heat and cold 

 as the latter. 



IV. SURFACE OF THE LAND. 



1. Inequalities of Surface. The inequalities of the earth's surface arising from mountains and 

 valleys, may be likened to the roughnesses on the rind of an orange compared with its general 

 mass. The highest mountain known does not exceed five miles in perpendicular height ; this 

 is only one 1600th part of the earth's diameter, and on a globe of 16 inches in diameter, such 



a mountain would be represented by a protuberance 

 of one hundredth part of an inch, which is about the 

 thickness of drawing-paper. The deepest mine 

 does not penetrate half a mile below the surface ; 

 and a scratch or pin-hole duly representing it on the 

 surface of such a globe, would not be perceptible 

 without a magnifier. The greatest depth of the sea 

 does not probably much exceed the greatest eleva- 

 tion of the continents, and the ocean would be re- 

 presented by a mere film of hquid, such as in our 

 model would be left by a brush dipped in color, and 

 drawn over those parts intended to represent the sea. 



2. Mountains. The greatest elevations of the 

 earth's surface are called mountains ; elevations of 

 an inferior height are called hills. Mountains are 

 View of the Mps. sometimes completely insulated, but thev are more 



4 



Whirlpool. 



