166 DYNAMICAL GEOLOGY. 



IV. WATER AS A MECHANICAL AGENT. 



(1) General sources of activity. — (a) Water does mechanical work in 

 each of its tliree states, the liquid, solid, and gaseous state (or that of vapor). 

 Only the first and second states are here considered, the third coming more 

 conveniently under the head of Heat. In the liquid state it constitutes 

 rivers, lakes, oceans; in the solid, snow, ice-crusts, glaciers, and icebergs. 

 Unlike the aerial ocean, it has a defined upper surface ; and the basement 

 on which it rests has usually no disturbing influence. 



(b) In rivers, water derives its energy from gravitation ; it ivorks as it 

 falls, and arrives at its zero of action on reaching the lowest level to which 

 it can fall. It reaches only temporary or approximate zeros in lakes, except 

 when the lakes are like the ocean in having no outlet. Winds make rela- 

 tively feeble currents and waves in large rivers. 



(c) In the ocean, water has three prominent working agencies: (1) the 

 tidal wave ; (2) the wind-waves and currents, both the regular winds, like 

 the trades, and the winds of storms, each producing waves and also currents 

 of greater or less depth and velocity; (3) the resupply currents caused by 

 the sun's heat, which in evaporation removes surface waters, and, in the 

 expansion of water, diminishes its density. Gravity acts toward a restora- 

 tion of the equilibrium that has been disturbed, whether the disturbance be 

 due to the tidal wave, wind-waves, currents, or heat, and in response also to 

 changes in atmospheric pressure. 



(d) Lakes of large size, like the ocean, have wind-made currents and 

 waves, and movements due to evaporation, and sometimes appreciable tidal 

 waves and currents. Those of small size are often only still-water incidents 

 in the courses of rivers. 



Winds over large rivers may slightly quicken, or retard, the flow. Over 

 great lakes, they may make decided onward movements, which pile the waters, 

 tide-like, on leeward shores, — as sometimes about Duluth at the western end 

 of Lake Superior, — occasioning an under current of escape. But over the 

 ocean they are in all parts a prominent source of currents, and in the 

 tropics, as has been stated, the " trade winds " originate, according to some 

 physicists, the Atlantic and Pacific tropical oceanic currents. 



(e) Oiving to the earth'' s eastward rotation, increasing in rate of surface 

 velocity from the pole to the equator as the cosine of the latitude, flowing 

 waters in the northern hemisphere, whether of rivers or the ocean, and 

 whatever their source, are thrown toward the right side as they advance, 

 and in the southern hemisphere toward the left side. The result is seen in 

 the lagging of the Labrador current against the west side of the north Atlan- 

 tic ; in a like effect on the correlate current in the north Pacific ; and in 

 the eastward course of the Gulf Stream north of the parallel of 35°. It has 

 also been observed a,long rivers in many parts of the world where the deposits 

 intersected are earthy, and the pitch of the stream is too small for erosion at 

 bottom. They are marked along the great rivers of Siberia and European 



