235 



LECTURE XXVI. 



ON HYDROSTATIC INSTRUMENTS AND HYDRAULIC 

 ARCHITECTURE. 



WE have now examined the fundamental laws of the principal depart- 

 ments 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, containing 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 raising and removing weights, in which the 

 principal hydraulic and pneumatic machines will be respectively explained, 

 and, as a part of this subject, the application of pneumatic force will also 

 be examined. 



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

 mination 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, affords 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 plung- 

 ing 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 proportion as 

 the specific gravities of the two fluids. A balance adapted for such exami- 

 nations is called a hydrostatic balance ; on one side it has a scale as usual, 

 and on the other a loop of fine wire or of horse hair, for holding the solid 

 to be weighed, which may be changed occasionally for a ball of glass, 

 suspended in a similar manner : sometimes also a dish is added for holding 

 any loose substances which will sink in water, proper counterpoises being 

 used as equivalents for the weight of the dish either in air or in water ; 

 and when a body lighter than water is examined, a weight of known 

 magnitude and density is employed for sinking it. (Plate XXI. Fig. 277.) 



The specific gravities of any substances, and in particular of such as are 

 lighter than water, may also be very conveniently determined by means of 

 a common balance, employing a phial with a conical ground stopple, filling 

 it first with water, and then either with a given fluid, or with a portion of 

 the solid of which the weight has been ascertained, together with as much 

 water as is sufficient to exclude all the air. 



