RIVER. 



ceed from snow, which, in some places, 

 is melted in the spring, in others, in sum- 

 mer, or between both, the deluges of 

 the rivers happen accordingly. Again, 

 some rivers hide themselves under 

 ground, and rise up in other place?, as if 

 they were new rivers. Thus the Tigris, 

 meeting with mount Taurus, runs under 

 it, and flows out at the other side of the 

 mountain : also, after it has run through 

 the lake Tospia, it again immerges, and 

 being carried about eighteen miles under 

 ground, breaks out again, &c. The 

 channels of rivers, except such as were 

 formed at the creation, Varenius thinks, 

 are artificial. His reasons are, that, when 

 a new spring breaks out, the water does 

 not make itself a channel, but spreads 

 over the adjacent land ; so that men were 

 necessitated to cut a channel for it, to se- 

 cure their grounds. He adds, that a 

 great number of channels of rivers are 

 certainly known from history to have been 

 dug by men. The water of most rivers 

 flow impregnated with particles of metals, 

 minerals, &c. Thus some rivers bring 

 sands intermixed with grains of gold ; as 

 in Japan, Peru, and Mexico, Africa, Cuba, 

 &C. particularly in Qumea is a river, 

 where the negroes separate the gold-dust 

 from the sand, and sell it to the Europe- 

 ans, who traffic thither for that very pur- 

 pose. The Riiine in many places is said 

 to bring a gold mud. As to rivers that 

 bring grains of silver, iron, copper, lead, 

 &c. we find no mention of them in authors, 

 though, doubtless, there are many, and 

 it may be to them that mineral waters owe 

 many of their medicinal virtues. 



Modern philosophers endeavour to re- 

 duce the motion and flux of rivers to pre-' 

 cise laws ; and with this view they have 

 applied geometry and mechanics to the 

 subject; so that the doctrine of rivers is 

 become a part of the new philosophy. 



The authors, who have most distinguish- 

 ed themselves in this branch, are the 

 Italians, and among them more especially 

 Gulielmini, and Ximenes. 



Rivers, says Gulielmini, usually have 

 their sources in mountains or elevated 

 grounds ; in the descent from which it is 

 mostly that they acquire the velocity, or 

 acceleration, which maintains their future 

 current. In proportion as they advance 

 further, this velocity diminishes, on ac- 

 count of the continual friction of the water 

 against the bottom and sides of the chan- 

 nel, as well as from the various obsta- 

 cles they meet with in their progress, and 

 from their arriving at length in plains, 

 where the descent is less, and conse- 

 VOL. V. 



quently their inclination to the horizon 

 greater. 



When the acquired velocity is quite 

 spent, through the many obstacles, so 

 that the current becomes horizontal, there 

 will then nothing remain to propagate the 

 motion, and continue the stream, but the 

 depth, or the perpendicular pressure of 

 the water, which is always proportional 

 to the depth. And this resource increases, 

 as the occasion for it increases ; for in 

 proportion as the water loses of the velo- 

 city acquired by the descent, it rises and 

 increases in its depth. 



It appears from the laws of motion, 

 pertaining to bodies moved on inclined 

 planes, that when water flows freely upon 

 an inclined bed, it acquires a velocity, 

 which is always as the square root of the 

 quantity of descent of the bed. But in 

 an horizontal bed, opened by sluices or 

 otherwise, at one or both ends, the water 

 flows out by its gravity alone. 



The greatest velocity of a river is about 

 the middle of its depth and breadth, or 

 that point which is the furthest possible 

 from the surface of the water, and from 

 the bottom and sides of the bed or chan- 

 nel. Where**, on the contrary, the least 

 velocity of the water is at the bottom and 

 sides of the bed, because there the resis- 

 ance arising from friction is the greatest, 

 which is communicated to the other parts 

 of the section of the river inversely as 

 the distances from the bottom and sides. 

 To find whether the water of a river al- 

 most horizontal, flows by means of the 

 velocity acquired in its descent, or by the 

 pressure of its depth, set up an obstacle 

 perpendicular to it ; then if the water rise 

 and swell immediately against the obsta- 

 cle, it runs by virtue of its fall ; but if k 

 first stop a little while, in virtue of its 

 pressure. 



Rivers, according to this author, almost 

 always make their own beds. If the bot- 

 tom have originally been a large declivity, 

 the water, hence falling witn a great force, 

 will have swept away the most elevated 

 parts of the soil, and carrying them lower 

 down, will gradually render the bottom 

 more nearly horizontal. 



The water, having made its bed hori- 

 zontal, becomes so itself, and consequent- 

 ly rakes with the less force against the 

 bottom, till at length that force becomes 

 only equal to the resistance of the bot- 

 tom, which is now arrived at a state of 

 permanency, at least for a considerable 

 time ; and the longer, according to the 

 quality of the soil, clay and chalk resist- 

 ing longer than sand or mud. 



4 G 





