Page:A Letter on the Subject of the Cause (1797).djvu/40

 piſton by an intermediate ſtrong bar of iron, in ſome cafes a ſtrong rod of wood ſhod with iron. By this means the force of the piſton received from the preſſure of the atmoſphere was communicated to the beam above, and that in as rough a manner as the workmen pleaſed to make it; the ſmoothneſs and truth of workmanſhip being unneceſſary in this caſe.

But only behold, my Lord, the difference required in Watt’s Engines in this one particular!

The above two motions are to be connected by means of a rod or other contrivances (for a chain, &c. will not anſwer here) which muſt not only paſs through an aperture in the cap or top of the cylinder, ſteam and air tight, bur this aperture is required to be kept thus cloſe during every ſtroke the Engine makes.

This cannot fail of ſtriking your Lordſhip in a ſerious point of view; and from what has been ſaid it muſt involve a concluſion in your mind, that this part is one grand eſſential, if not the moſt ſo, of any in the machine; as the ſmalleſt imperfection here will admit the air when the vacuum is made, and thereby completely ſtop the Engine. Having