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300 

 LECTURE XXV.. 



■ ON HYDRAULIC PRESST^rW. 



The mutual eflfects of fluids and moveable solids on each other depend 

 principally on the laws of hydraulic pressure, and of the resistance of fluids, 

 which have been considered by Bernoulli as constituting a separate depart- 

 ment of hydrodynamics, under the name of hydraulicostatics, and which are 

 of the utmost practical importance, since the application of the powers of 

 wind or water to the working of mills, and to the navigation of ships, are wholly 

 dependent on them. The impulse of a fluid differs very materially from that 

 of a solid, for in the motions of solids, the least possible finite momentum 

 must overpower the strongest possible pressure; but since the particles of 

 fluids are supposed to be infinitely small, the momentum of a fluid stream 

 may always be balanced by a certain determinate pressure, without producing 

 motion in the solid opposed to it; so that this division of the subject of hy- 

 draulics has nothing analogous to it in simple mechanics. It is true that 

 when a certain quantity of a fluid is made to concentrate its action almost 

 instantaneously, its effect is nearly similar to that of a solid; for here the 

 essential distinction, derived from the successive action of the particles, no 

 longer exists. Thus, when a stream of fluid filling a pipe acts suddenly on an 

 obstacle at the end of it, it requires to be resisted by a force far greater than 

 that which originally caused its motion, unless the action of the force be con- 

 tinued through a considerable space: and for this reason the strength of the 

 pipe ought to be so calculated as to be able to resist this action ; its inten- 

 sity may, however, be easily diminished by means of an air vessel com- 

 municating with the pipe, which will allow the motion to be changed in a 

 less abrupt manner. But in the principal cases Avhich we are about to consider, 

 the action of the fluid on the solid is supposed to be confined to such of its 

 particles as are nearly in contact with the surface. 



