ON HYDRAULICS AS A BRANCH OF EGINEERING. 159 



height of reservoh", the same quantity of water is always dis- 

 charged, whatever be the decUvity and length ; that the ve- 

 locities of the waters in the canal are not as the square roots 

 of the declivities, and that in equal declivities and depth of the 

 canal the velocities are not exactly as the quantities of water 

 discharged ; and he considers the variations which take place 

 in the velocity and level of the waters when two rivers unite, 

 and the manner in which they establish their beds. 



His experiments, in conjunction with D'Alembert and Con- 

 dorcet, on the resistance of fluids, in the year 1777, and his 

 subsequent application of them to all kinds of surfaces, in- 

 cluding the shock and resistance of water-wheels, have justly 

 entitled him to the gratitude of posterity. The Abbe Bossut 

 had opened out a new career of experiments ; but the most dif- 

 ficult and important problem remaining to be solved related to 

 I'ivers. It was easy to perform experiments with water running 

 through pipes and conduits on a small scale, under given and 

 determined circumstances : but when the mass of fluid rolled 

 in channels of unequal capacities, and which were composed of 

 every kind of material, from the rocks amongst which it accu- 

 mulated to the gravel and sand through which it forced a pass- 

 age, — at first a rapid and impetuous torrent, but latterly hold- 

 ing a calm and majestic course, — sometimes forming sand-banks 

 and islands, at other times destroying them, at all times capri- 

 cious, and subject to variation in its force and direction by 

 the slightest obstacles, — it appeared impossible to submit them 

 to any general law. 



Unappalled, however, by these difficulties, the Chevalier 

 Buat, after perusing attentively M. Bossut's work, undertook 

 to solve them by means of a theorem which appeared to him 

 to be the key of the whole science of hydraulics. He consi- 

 dered that if water was in a perfect state of fluidity, and ran in 

 a bed from which it experienced no resistance whatever, its 

 motion would be constantly accelerated, like the motion of a 

 heavy body descending an inclined plane ; but as the velocity 

 of a river is not accelerated ad infinitum, but arrives at a state 

 of uniformity, it follows that there exists some obstacle which 

 destroys the accelerating force, and prevents it from impressing 

 upon the water a new degree of velocity. This obstacle must 

 therefore be owing either to the viscidity of the water, or to 

 the resistance it experiences against the bed of the river ; from 

 which Dubuat derives the following principle : — That when 

 water runs uniformly in any channel, the accelerating force 

 which obliges it to run is equal to the sum of all the resistances 

 which it experiences, whether arising from the viscidity of the 

 water or the friction of its bed. Encouraged by this discovery, 



