Pressure of Fluids. 295 



power contained at the same time within a small compass, and 

 very simple in its construction. The diameter of the large pis- 

 ton being 1 2 inches, for example, and that of the small one work- 

 ed by the lever H and moving in AB, only one fourth of an inch, 

 the proportion of the two surfaces, or of the power employed to 

 the force exerted at E, would be as T ^ to 144, or as 1 to 2304. 

 Now it would be easy, by means of the lever HI, to apply to the 

 small piston a force equal to 20cwt. or one ton, in which case 

 the piston working in FD would be moved with a force of 2304 

 tons. This instrument is called the Hydrostatic or BramaWs 

 Press, Mr Bramah, an Englishman, being the first person who 

 made use of the hydrostatic principle here involved, as a substi- 

 tute for the screw in the construction of presses.! This machine 

 evidently belongs to the class of mechanical powers, and is es- 

 sentially different in its nature from those heretofore described. 

 The principle of virtual velocities, however, is equally applicable 

 to this ; since the greater the advantage gained in point of 

 intensity, just so much is lost in respect to velocity. Suppose, 

 for example, that the pipe AB, filled with fluid, is 2304 inches in 

 length, the small piston, by moving through this whole extent, 

 and thus forcing the entire contents of the pipe into the cylinder 

 FD, would raise the large piston only one inch ; so that while 

 the pressure upon the small piston is to that upon the large one, 

 in case of an equilibrium, as 1 to 2304, the spaces described in 

 the same time, or the velocities of the two pistons, are as 2304 

 to 1, and the quantity of motion in each is the same. 



411. Let there be two fluids NHCBFL, EFLM of different Fig.198. 

 densities, but each being homogeneous, considered by itself, and 

 let them be made to act against each other at FL by means of 

 the vessel in which they are contained. They can be in equi- 

 librium only when the altitudes EF, IK, above the horizontal 

 plane FL which separates them, are inversely as their specific 

 gravities. Indeed, the fluid LFBCGO being itself in equilibri- 497. 

 um, it is necessary that NHGO should be in equilibrium with 

 EFLM; it follows, therefore, that the upward pressure exerted 



t The same property of fluids is sometimes employed very advan- 

 tageously in a crane aud in raising water from mines. 



