762 



HYDKAULICS AND ITS APPLICATIONS 



between B and D being thus closed, the leakage from A into B allows the 

 pressure in the latter space to rise until it balances that in A , when the 



coil spring has sufficient 

 pressure to force valve 

 L down against its seat. 

 This construction evi- 

 dently has a number of 

 advantages. The use of 

 the small pilot valve, 

 the only member which 

 has to be lifted against 

 the full pressure of the 

 water, makes the appa- 

 ratus more easy to 

 handle than would be 

 the case if the whole 

 main valve L had to be 

 raised from its seat 

 against the full pres- 

 sure. Also there are no 

 sliding movements to 

 be effected through 

 ]_ packings under heavy 

 pressure, as the valve 

 stem H is not under 



FIG. 376. Hydraulic Operating Valves with Pilot Valves, pressure until the valve 



is opened. 



There would appear, also, to be little danger of shock from sudden 

 closing of the valves, since the lowering of main valve L to its seat on C 

 is effected by the leakage from A to B, which can be made as gradual as 

 seems advisable, being regulated by the fit of L in its cylindrical chamber 

 in the main casting 0. 



ART. 202. THE HYDRAULIC JACK. 



For the manipulation of heavy weights by hand the hydraulic jack is 

 of the greatest value. In principle it consists of a Bramah's press on a 

 small scale, and one type of its construction is illustrated in Fig. 377. 

 Here the reciprocation of the hand lever pumps water from the cistern 

 A, through the hollow plunger B, past the suction and delivery valves 



