152 WELLS'S NATURAL PHILOSOPHY. 



This fact is familiar to every one -who obsen^es tlie course of brooks or 

 rivers: wnerever the bed contracts, tlie current becomes rapid, and on tho 

 contrary if it widens, the stream becomes more sluggish. 



343. A very slight declivity is sufficient to give motion to 

 ^^*is'"*siiffi- running water. Three inches to a mile in a smooth, straight 

 ciont to give channel, gives a velocity of about three miles per hour. 

 j!]g'^^tg° "'°" The river Ganges, at a distance of 1,800 miles from it3 

 mouth, is only 800 feet above the level of the sea. Tho 

 i-vcrage rate of inclination of the surface of the Mississippi is 1.80 for the first 

 1 indred miles from the Gulf of Mexico, 2 inches for the second hundred, 2.30 

 lor the third, and only 2.57 for the fourth. 



^ . . The velocity of rivers is extremely variable; the slower class 



jiveraiTR vcloci- moving from two to three miles per hour, or three or four feet 

 ty of rivers ? pgj. ggcond, and the more rapid as much as six feet per second. 

 The mean velocity of the Mississippi, near its mouth, is 2.26 miles per hour, 

 or 2.95 feet per second.* 



The quantity of water which passes over the beds of rivers in a given time 

 is very various. In the smaller class of streams it amounts to from 300 to 

 350 cubic feet per second. In tho smaller class of navigable rivers, it amounts 

 to from 1,000 to 1,200 cubic feet; and in the larger class to 14,000 cubic feet 

 and upward. It is estimated that tho Mississippi discharges 12 billions of 

 cubic feet of water per minute.f 



• In the construction of water-channels for drainage, the regulation of inclination neces- 

 sary to produce free flowage of the water, is a matter of great importance. Thisinclinntion 

 varies greatly with the size of the stream of water to he conducted off. Large and deep 

 rivers run sufficiently swift with a fall of a few inches per mile ; smaller rivers and brooks 

 require a fall of two feet per mile, or 1 foot in 2,.500. Small brooks hardly keep an open 

 course under 4 feet per mile, or 1 in 1,200; while ditches and covered drains require at 

 least S feet per mile, or 1 in GOO. Furrows of ridges, and drains partially filled with loose 

 materials, require a much greater inclination. 



t A question of some interest relative to the course and flow of rivers, may, perhaps, 

 be appropriately considered in this connection. The question is as follows: Do the 

 Mississippi, .and other rivers whose courses are northerly and southerly, flow up hill or 

 down hill? The Mississippi runs from north to south. If its source were at the pole and 

 its mouth at the equator, the elevation of the mouth would be thirteen miles higher than 

 its source, as this is tlie difference between the equatorial and the polar radii of the 

 earth. On this principle, the mouth of the Mississippi is two and a half miles more ele- 

 vated than its source. Does it run up hill, and if so, how has its course and motion 

 originated? The problem, although apparently one of difficulty, admits of an easy 

 Bolution. 



The centrifugal force, caused by the rotation of the earth, has changed the form of our 

 planet from that of a perfect sphere to that of an ellipsoid, or a sphere flattened at the 

 poles, in which the length of the largest radius, e.-jceeds the shorter by thirteen miles, the 

 present form being the figure of equilibrium under the present conditions. The cohesion 

 of the solid particles of the earth has resisted, and does resist, to a limited e.xtent, 

 the influence of the centrifugal force which has changed the original figure ; but the par- 

 ticles of liquid on the eartli's surface, being perfectly free to move, yield to the influence, 

 and are at rest only so long as the condition of equilibrium is undisturbed, and always 

 move in such a way as tcf restore it when it is disturbed. Water, consequently, always 

 flows from places which are above the figure of equilibrium, to those which are below it. 

 Kow the mouth of the Mississippi is two and a half miles more distant from the center of 



