632 DYNAMICAL GEOLOGY. 



6. Changes of atmospheric pressure. — A local change of atmospheric 

 pressure from a passing storm has an effect on any large body of 

 water beneath it, a diminution of pressure causing the water di- 

 rectly beneath to rise from the greater pressure elsewhere. A 

 variation of one inch in the mercury column of a barometer is 

 equivalent to 13.4 inches in a column of water. Captain J. C. Ross 

 has observed in the Arctic regions that a change of pressure of this 

 kind was perceptible in the tides. Observations through forty-seven 

 days gave a variation in the water of nine inches, corresponding to 

 two-thirds of an inch in the barometer. 



The wind during storms produces sometimes an elevation of the 

 water in the leeward part of a lake at the expense of that in the 

 other, as has often been observed in the great lakes of North Ame- 

 rica. Great waves on the ocean and extraordinary tides on sea- 

 coasts are other effects of the same cause. The subject of waves is 

 treated of under the head of Water. 



IV. WATER. 



Subdivisions of the subject. 



1. Fresh waters ; including especially Rivers and the smaller 

 Lakes. 



2. The Ocean ; including the larger lakes, whether salt or fresh- 

 water, — the general facts being similar, excepting such as depend 

 On the tides, and the kind and density of the water. 



3. Frozen waters, or Glaciers and Icebergs. 



1. FRESH WATERS. 



The Superficial waters and the Subterranean may be separately 

 considered. 



A. SUPERFICIAL WATERS, OR RIVERS. 

 1. General Observations on Rivers. 



1. Water of rivers. — The fresh waters of the land come from the 

 vapors of the atmosphere, and these are largely furnished by the 

 ocean. They rise into the upper regions of the atmosphere, and, 

 becoming condensed into drops, descend about the hills and plains, 

 and so begin their geological work, — gravity being the moving 

 power. 



The amount of water in a river depends on (1) the extent of the 



