CHARACTERISTICS OF WATER 71 



but in a few places it sinks below, as in the Salton Sea, 

 a part of Imperial Valley, California, and near the Dead 

 Sea. Its surface is eroded by wind and water and is thus 

 constantly but slowly changing its features. Travel upon 

 the land, over most of the earth's surface, is difficult be- 

 cause of the irregularities. 



SALT WORKS ON THE SHORE OF THE SALTON SEA. 



In 1905 the Colorado River broke through into this depression, which is below 

 sea level, and completely covered the salt works seen in the picture. 



Surface conditions also vary greatly over small areas. 

 Great temperature changes occur, on the land between day 

 and night and between summer and winter. Land animals 

 must exert considerable muscular force to move about, 

 yet they must all move to get their food. They must 

 therefore be highly organized to maintain themselves upon 

 the land. Water animals are not subjected to the same 

 difficult conditions. In fact, the conditions of life on sea 

 and land surfaces are entirely different. 



39. Characteristics of Water. Experiment 36. Place in a dish 

 of fresh water a density hydrometer, or stick loaded with lead at one 

 end, so that it will float upright. Mark with a rubber band the point 



