61:0 DYNAMICAL GEOLOGY. 



the contrary, the velocity of a stream does not increase uniformly as 

 it descends, and when it reaches the sea, whatever the elevation at 

 first, its velocity is in most cases nearly zero. This is owing to the 

 fact that its energy, instead of being stored up, is being expended 

 against the various resistances encountered, that is: — 



(1.) In overcoming friction between (a) the molecules of the water 

 itself; (b) the water and the bed of the stream; (c) the surface of the 

 water and the atmosphere. 



(2.) In impact, or blows against the rocks or earthy material of the 

 bed and banks of the stream ; and in pushing sand or gravel along the 

 bed. 



(3.) In transporting earth, sand, or stones, held in suspension in 

 the water. 



(4.) In overcoming the friction between the transported particles 

 and the bed of the stream, and the friction between the particles them- 

 selves ; and also the loss from eddies made by the character or form 

 of the bed or otherwise. 



By these means the energy is so far used up, that no accumulation 

 can take place except on portions of a stream where the pitch is uni- 

 form and considerable, and the bed is hard and smooth. In a water- 

 fall accumulation goes on during the descent ; but the whole energy 

 of the stream is lost in the stroke of the water at the bottom of the 

 fall, where it is converted into heat, — a fall of 772 feet producing 

 heat enough to raise the temperature of the water 1° F. 



Owing to the rapid increase of velocity in the descending water of 

 a waterfall, the stream in a high fall of small volume becomes divided 

 up, the parts running away from one another and finally separating 

 into drops; in which case, owing to the resistance of the air, the ve- 

 locity, and therefore the energy, is almost wholly dissipated, and the 

 fall becomes a veil of mist, swayed by the winds. 



The amount of actual work which a stream is capable of doing at any point in its flow 

 increases with the mass of the water and the square of its mean velocity, its value being 



given by the formula — — already stated. The greatest velocity will be near the 



middle of the stream, or as remote as possible from the surfaces of friction; and, owing 

 to the friction with the atmosphere, below the upper surface. The vertical curve of sub- 

 surface velocities, according to Humphrey's and Abbot's careful observations on the 

 Mississippi (confirmed by General Ellis's on the Connecticut) is parabolic; the axis, or 

 line of greatest velocity, is about one-tenth of the depth below the surface; and the ve- 

 locity at mid-depth equals almost uniformly 0.955 the mean velocity ; a wind up stream 

 depresses it slightly, and one down stream raises it. 



Other characteristics of rivers are brought out in the following 



