300

LECTURE XXV.

ON HYDRAULIC PRESSURE.

r I^IlE mutual effects of fluids and moveable solids on each other dependprincipally 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 areof the utmost practical importance, since the application of the powers otwind or water to the working of mills, and to the navigation of ships,are whollydependent on them. The impulse of a fluid differs very materially from thatof a solid, for in the motions of solids, the least possible finite momentummust overpower the strongest possible pressure; but since the particles otfluids are supposed to be infinitely small, the momentum of a fluid streammay always be balanced by a certain determinate pressure, without producingmotion 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 thatwhen a certain quantity of a fluid is made to concentrate its action almostinstantaneously, its effect is nearly similar to that of a solid, for here theessential distinction, derived from the successive action of the particles, n°longer exists. Thus, when a stream of fluid filling a pipe acts suddenly on anobstacle at the end of it, it requires to be resisted by a force far greater thanthat 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 lb cpipe 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 aless abrupt manner. But in the principal cases which wc are about to consid el »the action of the fluid on the solid is supposed to be confined to such ofparticles as are nearly in contact with the surface.