Page:A Letter on the Subject of the Cause (1797).djvu/60

 Now let it be ſuppoſed, that the uncondenſible air or vapour, which then fills the condenſer, and is to be drawn out by a pump, is equal in expanſive force to one twelfth part (and it is ſeldom leſs) of the ſteam’s preſſure on the piſton. The air pump, which I have already ſaid is only one twelfth part of the contents of this condenſer, makes one ſtroke alſo; but by this the expanſive force of the vapour can only be reduced one twelfth part; for it muſt take twelve ſtrokes of this pump to reduce the vapour in the condenſer to its leaſt denſity; and conſequently there will remain a reſiſtance to the ſecond ſtroke equal to of the force of the vapour mentioned; and to the next ſtroke  and every continued ſtroke in this proportion; ſo that in about thirteen ſtrokes, this air and vapour would inevitably become as ſtrong in the condenſer as the ſteam; and by thus reſtoring the equilibrium, of neceſſity ſtop the Engine,—although ſhe had nothing but her own materials to carry, and thoſe void of friction.

Here your Lordſhip muſt ſee, that from the confidence I had been induced to place in the abilities of Mr. Watt, from what I ſaw at Birmingham, and heard in the Courts above-mentioned, I have adopted, in this ſuppoſed Engine conſtructed