Page:A Letter on the Subject of the Cause (1797).djvu/68

 proportionate defect. Yet your Lordſhip was told by Mr. Cummings, that in order to counteract the extraneous ſize of the pump, when it happened to require it, the ſtroke of the piſton could eaſily be altered in its length by any experienced workman, ſo as to adjuſt any ordinary want of proportion between the ſaid pump and the condenſer; and he that could not do it muſt be a blockhead. I think your Lordſhip will perceive, that by doing this, the piſton muſt fall ſhort of reaching one end of the pump cylinder at any rate; and if otherwiſe muſt fall ſhort of both, and thus occaſion a defect in the power of the Engine. So much for the invention of Mr. Cummings.

There is one point more, to which I would intreat your Lordſhip’s attention; it being a matter of great import in the fabrication of Engines on this plan. This is what is called a ſtop-back valve in Hydraulics; and is one that opens by the current of any fluid in one direction, and ſhuts by its own gravity to prevent a return the ſame way; for the ſame office as thove in the blood-veſſels of animals. A valve of this deſcription is placed in the communication between the condenſer and the air-pump, in order to admit the condenſed water and air into the bottom of the pump-cylinder from the condenſer; and to