MECHANICS, 



the number of revolutions of the wheel 

 performed in a given time may be thus 

 found. Another rectangular index m n p 

 shows the parts of a revolution. At the 

 commencement of the motion, the point 

 p is directed to on the graduated rim 

 of the wheel. 



Having found by this instrument the 

 number of revolutions and fractional 

 parts of a revolution which have been 

 performed in a given time ; multiply 

 the circumference of the wheel by that 

 number, and we shall then find the ve- 

 locity with which the circumference of 

 the wheel moves. 



(52.) The third property, in virtue 

 of which water becomes a' mechanical 

 agent, is that power which, in common 

 with all fluids, it possesses of transmit- 

 ting pressure equally in every direction. 

 If water be confined in any vessel, and 

 a pressure to any amount be exerted on 

 a square inch of that water, a pressure 

 to an equal amount will be transmitted 

 to every square inch of the surface of 

 the vessel in which the water is con- 

 fined. 



One of the most remarkable instances 

 of the employment of this property as a 

 mechanical agent, is in Bramah's hy- 

 drostatic press, the theory of which is 

 extremely simple. A large solid plug or 

 piston A B (Jig. 25.) is constructed so as 

 to move water-tight in a cylinder C D. 

 The space beneath the piston is filled 

 with water, and communicates by a pipe 

 E F with a small forcing-pump, worked 

 by the piston G,and by which the water is 

 forced into the chamber of the cylinder 



ftg- 25. 



C D below the great piston. Let us now 

 suppose the entire space between the 

 two pistons to be filled with water, and 

 a pressure of one pound exerted on the 

 water by means of the piston G of the 

 forcing-pump. Let us also suppose that 

 the diameter of the piston G is a quarter 

 of an inch, and that the diameter of the 

 piston B is one foot. In that case, the 



base of the piston ^^-v > 



by the water, is 2304 i&i^ th%se <Jf f 7> ^ 

 the piston G, which pre^eirmftiw,ter, 4 

 and in virtue of the power of tj1Hq3tJ A- s 

 ting pressure to which we have Already 

 alluded, a pressure of one pound will be 

 transmitted to every part of the base of 

 the greater piston which is equal to the 

 base of the less. Thus an urging pressure 

 of one pound on the base of the lesser 

 piston G will produce a pressure of 

 2304/fo. against the base of the greater 

 piston B. This property of fluids, there- 

 fore, seems to invest us with a power of 

 increasing the intensity of a pressure 

 exerted by a comparatively small force, 

 without any other limit than that of the 

 strength of the materials of which the 

 engine itself is constructed. 



This property of liquids also enables 

 us with great facility to transmit the 

 motion and force of one machine to 

 another, in cases where local circum- 

 stances preclude the possibility of insti- 

 tuting any ordinary mechanical con- 

 nexion between the two machines. Thus 

 merely by means of water-pipes the 

 force of a machine may be transmitted 

 to any distance, and over inequalities of 

 ground, or through any other obstruc- 

 tions. 



CHAPTER VI. Air considered as a 

 Mechanical Agent. 



(53.) AIR may become a mechanical 

 agent by means of its four properties, 

 weight,' inertia, fluidity, or power of 

 transmitting pressure, and its elasticity. 



In our treatise on PNEUMATICS, Chap- 

 ter III., it was proved, that a column of 

 air, whose base is one square inch, and 

 whose height is that of the atmosphere, 

 weighs about fifteen pounds. Conse- 

 quently, it follows, that an horizontal 

 surface sustains a weight or pressure 

 amounting to fifteen times as many 

 pounds as there are square inches in its 

 extent. If then we have a solid sub- 

 stance with an horizontal surface, for 

 example, a piston placed in a vertical 

 cylinder, and that we are able by any 

 means to remove all resistance from 

 below it, it will be forced down by a 

 mechanical pressure of fifteen times as 

 many pounds as there are square inches 

 in its upper surface, and in this way a 

 mechanical agent of a power limited 

 only by the magnitude of the piston will 

 be obtained. 



But peculiar difficulties in giving 

 efficacy to this power arise from two 



