PHYSICAL GEOGRAPHY. 103 



its immediate locality. The waters of the Tropical regions 

 are generally salter than those towards the Poles ; but, 

 during the monsoons on the coast of Malabar, the water 

 becomes so fresh as to be almost drinkable. In some 

 places the water is salter at the ebb of the tide than at 

 the flow; in others, the contrary takes place. These 

 phenomena are, however, but rare. The saltness in- 

 creases the density, and consequently the buoyancy of 

 salt water compared with river water, and this renders 

 it advantageous for the purpose of navigation.* 



The temperature of the water on the surface is much 

 the same as that of the atmosphere, but it has been 

 found to decrease as the depth increases. At the depth 

 of 3SO fathoms near the Equator the temperature was 

 found to be only about 45 of Fahrenheit, when at the 

 surface it was 80. 



The chief phenomenon attending the sea is its Tides. 

 The advantage of the tides is too apparent to require 

 noting. The water, by its constant movement, is pre- 

 vented from becoming putrid ; and vessels, by means of 

 the rise of the tide, are enabled to enter rivers and har- 

 bours, and land their freight much more conveniently 

 than otherwise they would be able to do. 



The ancients had no correct idea of the cause of the 

 tides, but the moderns know that they are caused by 

 the combined action of the sun and moon. 



Independent of the solar and lunar action, there is 

 another cause which produces a motion of the water in 

 various parts. This motion is called a current. The 

 principal current is that within the TrDpics, and its di- 

 rection is contrary to that of the rotation of the Earth, 

 viz., from East to West. A general current also flows 

 from the Poles towards the Equator, which is caused by 

 the increased evaporation in the Torrid Zone. 



It is the business of navigators to ascertain the nature 

 and drifts of the various currents, and to take advantage 

 of them. 



Whirlpools are occasionally met with in the ocean. 

 These are sometimes caused by the congruence of oppo- 



* Some years since a vessel heavily laden, after having crossed the 

 Atlantic in safety, sank when it entered the Thames. 



