Military Bridges, and the Passage of Rivers, 289 



in their conjoint experimental course. The whole forms an in- 

 structive specimen of the method of discovering the laws of na- 

 ture, involved in a series of complicated phenomena. The Che- 

 valier Du Buat's first experiments were undertaken to establish 

 the similarity of the motion of water in pipes and open canals. 

 From these he proceeds to show that the moving principle of 

 water arises from gravity and the mobility of the particles, 

 which causes the fluid to assume a level in close vessels, or de- 

 termines it to move to that side where there is a defect of pres- 

 sure ; and, excepting in very small tubes, there is scarcely any 

 declivity so small that water will not move in it, — are all owing to 

 the same cause. From this reasoning Du Buat derives his first 

 principle, namely, that the motive force of each particle of water 

 composing a river arises simply from the slope of the surface. 



Again, if a current of water were not resisted by the bed in 

 which it runs, and if its fluidity were perfect, its velocity would 

 become more and more accelerated, to such a degree that its 

 destructive impetuosity would become irresistible. But the 

 friction of the water against the bed of the river, which, by the 

 effect of adhesion, is communicated to the whole mass of fluid, 

 causes a resistance, which, augmenting nearly as the squares of 

 the velocity, at length equals the accelerating force, when the 

 velocity will remain uniform, without the possibility of increas- 

 ing, unless a change takes place in the slope of the river, or in 

 the dimensions of the section. 



From the foregoing reasoning, Du Buat obtains his second 

 principle, namely, that when a stream moves uniformly, the re- 

 sistance is equal to the accelerating force. 



It follows from observation, combined with reasonings on the 

 nature of rivers, that, in open canals and rivers, the greatest 

 velocity is at the surface in the middle of the stream, and in 

 close pipes in the central line or axis ; because there the moving 

 water is most remote from all causes of resistance. If this velo- 

 city, therefore, be taken to calculate the force or discharge of 

 the water, the results will, in both cases, be erroneous in excess. 

 The situation of the line or stream of mean velocity in the sec- 

 tion varies with so many circumstances, that it cannot be pre- 

 cisely ascertained ; though, by a series of very ingenious experi- 



VOL. XIII, NO. XXVI. OCTOBER 1832. T 



