MB 



APPLIED MECHANICS. 



[TUB HYDRAULIC CHANS. 



with relation to that on which it acts, springs must be 

 employed. And so for the regulation of portable ap- 

 paratus, escapements in which the elasticity of a spring 

 IB used for recoil, instead of the weight of a pendulum, 

 must be likewise employed. The details of all such ap- 

 paratus, and the many ingenious contrivances applied to 

 them, form the subject of . ;irt, that of Horology. 



^-1 l:i: AM) ELASTICITY. Beside* contri- 

 vances for employing the weight and elasticity of solid 

 bodies in giving motion to machinery, there are others 

 by which the weight or pressure of liquids and the elas- 



y of gases are also applied for such purposes. 

 Among these the hydraulic lift and die hydraulic 

 crane may be particularly noticed. The hydraulic lift, 

 iu its most simple form, consists of a cylinder closed at 

 bottom, and fitted with a plunger or ram, which passes 

 through the top and supports a stage (Fig. 139). A 

 Fig. 139. 



leather collar, fitting round the plunger, is placed in a 

 recess provided in. the neck of the cylinder, so as to 

 prevent the escape of water around the plunger. A 

 pipe from a high cistern conducts water into the 

 cylinder, and another pipe permits the water to issue 

 from it. Each of these pipes is provided with a stop- 

 cock, so that the water may be admitted to the cylinder, 

 or allowed to flow from it at pleasure. Since liquids 

 communicate pressure equally in all directions, every 

 part of the cylinder and the plunger is pressed upon, by 

 a force proportioned to the height of the cistern which 

 supplies the apparatus. Every 27 inches of height pro- 

 duces a pressure of 1 Ib. on every square inch of surface 

 exposed to it ; and by making the area of the bottom of 

 the plunger sufficiently Urge, a considerable weight can 

 be raised on the stage whioh it carries. Thus if the 

 cistern at the top of a house be 54 feet above the 

 ground-floor, the pressure on the internal surface of any 

 vessel on the ground. floor connected witli it liy a pipe, is 

 24 Ibs. per square inch. If this vessel be a cylinder, 



such as we have described, fitted with a plunger 10 inches 

 diameter, having therefore a sectional an u:iro 



inches, the pressure forcing this plunder np.vards 

 amounts to 7J, X 24 1884 Ibs., and if from this we 

 deduct 764 Ibs. as a weight including that of the pinn-.T 

 and platform, there remains a pressure of 1120 Ibs. 

 forcing the plunger upwards. A load then of this 

 amount, half a ton, may be placed on the platform 

 when it is in its lowest position, ami the cock being 

 opened to the cistern, the whole will be raised to a 

 height equal to that of the plunger, which may be made 

 Miilirirnt for lifting goods from one floor to another. By 

 closing the cistern-cock and opening the other, the water 

 is permitted to leave the cylinder ; and the plunger, no 

 longer subjected to upward pressure, descends to its 

 former position. 



The water-pressure may be made to act in a way some- 

 what different in detail, but similar hi principle, by fitting 

 the cylinder with a piston, connected by a rod passing 

 tightly through the cylinder cover with a rope or chain, 

 which may be led by pulleys in any convenient direction 

 for lifting weights attached to it. The water- pressure 

 acting on the piston, forces it down the cylinder, and 

 thus pulls the chain and lifts the weight. Such an 

 arrangement constitutes the hydraulic crane. The ad- 

 Fig. 140. 



vantage of employing water for lifting weights in this 

 manner consists in the circumstance that a small steam- 

 engine, or other power, can be constantly ernployril iu 

 raising the water to a high cistern, and that great lifting 

 power can be obtained for a short time by the expendi- 

 ture of a portion of the water thus raised. The water 

 lifted to a height, becomes, in fact, a reservoir of po IT, 

 ready to be used when required, and accumulates the 

 efforts of a small power acting through a considerable 

 period, ready to be expended as a great power acting 

 through a short time. Farther, this power is completely 

 under control ; for by the mere turning of a stop-cock, 

 or the opening or closing of a valve, it can be put in 

 action or arrested at pleasure ; and the speed with which 

 a weight is moved, and the height to which it is raised, 

 can be regulated with the greatest nicety. 



Apparatus has sometimes been applied in which 

 water flowing from a high level, or compressed air, may 

 be made to act as steam in a steam-engine, for giving 

 motion to machinery. The construction of such appara- 

 tus is very similar to that of the steam-engine, and may 

 be easily understood by one who has studied the arrange- 

 ment of the hitter. 



The pressure of the atmosphere has also been era- 

 ployed as a moving force in the case of the atmospheric 

 railway. 



4. HEAT AND ELECTRICITY. In treating of 

 heat, electricity, magnetism, and chemical action, as 

 sources of power, we embrace a very wide range of 



