308 



LECTURE XXVI. 



ON HYDROSTATIC INSTRUMENTS, AND HYDRAULIC 



ARCHITECTURE. 



At E have now examined the fundamental laws of the principal departments 

 of hydrodynamics, which may be considered as constituting the theory of the 

 science: we are next to proceed to the application of this theory to a variety 

 of practical purposes. Following the same general order as we have observed 

 in mechanics, our first division will be analogous to the subject of statics, 

 and will relate to hydrostatic instruments ; the second to architecture, con- 

 taining some particulars respecting canals and embankments; the third to 

 machinery, comprehending the modification and application of the force of 

 fluids considered as inelastic; the fourth and the fifth to the methods of rais- 

 ing and removing weights, in which the principal hydraulic and pneumatic 

 machines will be respectively explained, and,as a part of this subject, the appli- 

 cation of pneumatic force will also be examined. 



The principles of hydrostatics are very frequently applied to the determi- 

 nation of the specific gravities of the various productions of nature or of art. 

 The diminution of the apparent weight of a solid body, upon immersion into 

 a fluid, affbrds an easy method of comparing its density with that of the fluid. 

 For the weight of the solid being previously determined, if we examine how 

 much that weight iS diminished by plunging the body in pure water, we shall 

 have the weight of an equal bulk of water; and thence we may immediately 

 obtain the proportion of the specific gravity of the body to that of water, 

 which is the usual standard of comparison. And if we weigh a solid of 

 given magnitude, for instance, a ball of glass, first in water, and then in any 

 other fluid, the quantities of weight lost in each case will be in the same pro- 

 portion as the specific gravities of the two fluids. A balance adapted for such 

 -examinations is called a hydrostatic balance; on one sidfi it has a scale as 



