THE NON-LIVING WORLD 153 



course, of the same height in each limb when there is no 

 pressure. Then when the horizontal part of the tube is 

 made to face the wind, its pressure will be registered by 

 the amount of elevation thus produced, of the water, in 

 the opposite limb of the tube. A gentle breeze will 

 support 0.025 inches of water, a brisk gale, 0.5. A storm 

 will sustain 3 inches, and a violent hurricane, 9 inches. 



There are certain elevations and depressions which 

 affect the whole aerial envelope of the globe, but these 

 will be noticed in connection with " Ocean Tides." * 



Water is commonly spoken of as being " fresh " or 

 "salt" water. In fact, however, it always contains a 

 greater or less quantity of foreign substances dissolved 

 in it, but not, of course, chemically united with it. In 

 the first place water except water that has been boiled 

 contains a considerable quantity of air mixed up 

 within it, and rain-water gathers in its descent, some of 

 the air's soluble constituents, including carbonic acid and 

 ammonia. The water of each river necessarily contains 

 some of the salts of the springs which feed it, and it also 

 contains the matters which it dissolves out from the 

 material which it meets with in its course. Amongst 

 the more noteworthy ingredients it thus acquires, are 

 carbonate of lime and flint in a state of solution. Thus 

 the Thames carries past Kingston daily not less than 1514 

 tons of solid substance (mainly derived from calcareous 

 formations of Berkshire, Oxfordshire, and Gloucester- 

 shire), which includes more than 1000 tons of carbonate 

 of lime. Sea-water notoriously contains a great deal of 

 salt, with other chlorides and sulphates, and with some 

 ammonia and iodine. 



There are currents in the ocean due to differences of 



, p. 182. 



