RIVER. 



On the other hand, the water is conti- 

 nually wearing away the brims of its chan- 

 nel, and this with the more force, as, by 

 the direction of its stream, it impinges 

 more directly against them. By this 

 means it has a continual tendency to ren- 

 der them parallel to its own course. At 

 the same time that it has thus rectified 

 its edges, it has widened its own bed, 

 and thence becoming less deep, it loses 

 part of its force and pressure : this it 

 continues to do till there is an equilibrium 

 between the force of the water and the 

 resistance of its banks, and then they 

 will remain without further change. And 

 it appears, by experience, that these 

 equilibriums are all real, as we find that 

 rivers only deepen and widen to a certain 

 pitch. 



The union of two rivers into one makes 

 the whole flow the swifter, because, in- 

 stead of the friction of four shores, they 

 have only two to overcome, and one bot- 

 tom instead of two ; also the stream be- 

 ing further distant from the banks, goes 

 on with the less interruption ; besides, 

 that a greater quantity of water, moving 

 with a greater velocity, digs deeper in the 

 bed, and of course retrenches of its for- 

 mer width. Hence also it is, that rivers 

 by being united, take up less space on 

 the surface of the earth, and are more 

 advantageous to low grounds, which 

 drain their superfluous moisture into 

 them, and have also less occasion for 

 dykes to prevent their overflowing. 



A very good and simple method of 

 measuring the velocity of the current of 

 a river, or canal, is the following. Take 

 a cylindrical piece of dry light wood, and 

 of a length something less than the depth 

 of the water in the river ; about one end 

 of it let there be suspended as many 

 small weights, as may keep the cylinder 

 in a vertical or upright position, with its 

 head just above water. To the centre of 

 this end fix a small straight rod, precise- 

 ly in the direction of the cylinder's axis ; 

 to the end that, when the instrument is 

 suspended in the water, the deviations of 

 the rod from a perpendicularity to the 

 surface of it, may indicate which end of 

 the cylinder goes foremost, by which may 

 be discovered the different velocities of 

 the water at different depths ; for when 

 the rod inclines forward, according to the 

 direction of the current, it is a proof 

 that the surface of the water has the 

 greatest velocity ; but when it reclines 

 backward, it shows that the swiftest cur- 

 rent is at the bottom ; and when it re- 

 mains perpendicular, it is a sign that the 



velocities at the top and bottom are equal. 

 This instrument, being placed in the cur- 

 rent of a river or canal, receives all the 

 percussions of the water throughout the 

 whole depth, and will have an equal ve- 

 locity with that of the whole current from 

 the surface to the bottom at the place 

 where it is put in, and by that means may 

 be found, both with exactness and ease, 

 the mean velocity of that part of the river 

 for any determinate distance and time. 

 But to obtain the mean velocity of the 

 whole section of the river, the instru- 

 ment must be put successively both in 

 the middle and towards the sides, because 

 the velocities at those places are often 

 very different from each other. Having 

 by this means found the several veloci- 

 ties, from the spaces run over in certain 

 times, the arithmetical mean proportional 

 of all these trials, which is found by di- 

 viding the common sum of them all by 

 the number of the trials, will be the 

 mean velocity of the river or canal. 

 And if this medium velocity be mul- 

 tiplied by the area of the transverse 

 section of the waters at any place, the 

 product will be the quantity running 

 through that place in a second of time. 



If it be required to find the velocity of 

 the current only at the surface, or at the 

 middle, or at the bottom, a sphere ot 

 wood loaded, or a common bottle corked 

 with a little water in it, of such a weight 

 as will remain suspended in equilibrium 

 with the water at the surface or deptli 

 which we want to measure, will be better 

 for the purpose than the cylinder, be- 

 cause it is only affected by the water oi 

 that sole part of the current where it re- 

 mains suspended. 



It follows from* what has been said in 

 the former part of this article, that the 

 deeper the waters are in their bed in 

 proportion to its breadth, the more their 

 motion is accelerated ; so that their velo- 

 city increases in the inverse ratio of the 

 breadth of the bed, and also of the mag- 

 nitude of the section; whence, in order 

 to augment the velocity of water in a 

 river or canal, without augmenting the 

 declivity of the bed, we must increase the 

 depth of the channel, and diminish its 

 breadth. And these principles are agree- 

 able to observation ; as it is well known., 

 that the velocity of flowing waters de- 

 pends much more on the quantity and 

 depth of the water, and on the com- 

 pression of the upper parts on the lower, 

 than on the declivity of the bed ; and 

 therefore the declivity of a river must be 



