332 PHYSIOGRAPHY 



a thermometer made for this especial purpose. Its chief peculiari- 

 ties are (1) that it will stand the great pressure of deep water, and 

 (2) that it will record the temperature of any depth. 



The ice of the sea has been referred to in other connections (p. 

 102). The movement of floating ice is controlled partly by the 

 winds, and partly by the movements of the water in which the ice 

 floats. 



THE MOVEMENTS OF SEA-WATER 

 Causes 



We have seen that differences in saltness and in temperature 

 make water unequal in density, and that differences in density pro- 

 duce a slow circulation of the waters of the sea. There are other 

 causes, also, which produce movement. Among them are (1) dif- 

 ferences of level, (2) winds, and (3) the attraction of the moon 

 and the sun. There are also (4) occasional causes, such as earth- 

 quakes and volcanic explosions, which produce sudden, and in some 

 cases disastrous, movements. 



Movements due to the inequalities of level. The inequalities 

 of level which produce movement are brought about chiefly by 

 (1) the discharge of rivers, which raises the surface of the sea at the 

 point of inflow; (2) winds, which pile up the waters along the 

 shores against which they blow; (3) unequal rainfall, which raises 

 the surface most where most falls; and (4) unequal evaporation, 

 which lowers the surface most where it is greatest. 



Inequalities of level of the surface cause movements of its 

 waters. The movements due to unequal rainfall and evaporation 

 are generally too slight to be seen or felt. Those caused by the 

 inflow of rivers and by the wind are greater. Thus, beyond the 

 mouth of a great river like the Amazon, the movement is often dis- 

 tinct for many miles, and. waters are often piled up against a shore 

 by winds, to such an extent as to be readily seen. During a storm 

 on the coast of India in 1864, the water was raised 24 feet at Cal- 

 cutta, drowning 48,000 people. The raising of the surface of the 

 water caused most of the destruction in the storm at Galveston, 

 already referred to (p. 295). When the water level has been raised 

 along a coast by the wind, it will settle back after the wind goes 



