THE STEA M-ENGINE. ] 65 



judged of by the 140 or 150 feet of resistance of head actually 

 required ; and moreover there is no necessity for the purposes 

 of economy to have one of these machines, which, in addition 

 to the difficulties I have mentioned as attending its applica- 

 tion to the supplying of distributing mains, has the defect of 

 being unlimited in its range, that is of having no determinate 

 stroke. With a double-acting engine working upon a crank 

 the length of the stroke is determined, and leading into an 

 air vessel or into a single stand-pipe (not a double one, going 

 up and coming down again) such an engine is resisted solely 

 by the load which actually prevails in the mains, and the 

 work that engine is doing is real, whereas a Cornish machine 

 requires a constant maximum pressure, although the mains 

 may not require that constant maximum pressure. I say, 

 therefore, that is an instance of how the engineer is misled 

 by fashion. There are cases where single stand-pipes are used 

 with Cornish engines, and there you will actually see throttle- 

 cocks put on to the foot of the stand-pipe to keep the "head " 

 up, and throttle- valves put in the equilibrium-pipe to prevent 

 these engines from having only their right amount of work 

 to do when that amount of work happens to be too small 

 to balance the working weight. 



To revert to the question of expansion ; in the Cornish 

 engine the steam is cut off by the shutting of a double beat 

 valve (the valve which Harvey and West have introduced 

 also into their pump work), and we have also in slow-going 

 engines valves lifted by cams and dropping at proper 

 intervals. We have then in quick running engines, the 

 Corliss valve moved by an eccentric and brought back to rest 

 by a pneumatic cylinder (which on account of the risk of an 

 air leak, I cannot help thinking is a dangerous thing, I 

 would rather see a steam cylinder than a pneumatic one). 

 The Corliss valves are liberated from the eccentric rod by a 

 spring catch connected with the governor. But one of the 

 commonest modes of making expansion in our ordinary 

 engines, is to have one slide valve upon the back of the 

 other, each valve worked by its own eccentric. By that 

 contrivance it is possible to cut off at almost any part of the 

 stroke. There is no doubt a certain want of sharpness in 

 the cut off, and you do not get on the indicator that beau- 

 tiful-looking figure which you do by the Corliss valve, or the 

 drop valve of any kind, but when one comes to work it out, 



