The Century of Inventions of the Marquis of Worcester / from the Original MS., with Historical and Explanatory Notes and a Biographical Memoir

by Marquis of Edward Somerset Worcester

Having thus produced a vacuum without the intervention of condensing water beneath the piston, Mr. Watt's next improvement consisted in closing the top of the cylinder, so that the piston-rod worked through an air-tight hole in the centre of the cap; and to ensure the necessary pressure within the cylinder, steam with an elastic force greater than that of the atmosphere was admitted above the piston. The atmospheric engine of Newcomen was thus converted into a steam-engine, and its power was easily regulated.

A cylinder and piston constructed on the most improved principles may now be examined.

In the annexed diagram, the cylinder A is furnished with a steam-tight piston, the rod of which is supposed to be connected with the working beam. B represents the pipe which admits the steam from the boiler, the quantity being regulated by the throttle valve c, and the elastic vapour is now passing through the box d d, so that it enters beneath the piston. At the same instant of time, a communication is formed through the aperture m n to the pipe p, which leads to the condenser. When the piston reaches the top of the cylinder, the sliding bridge or valve has its direction changed, so that the pipe r, and consequently the bottom of the cylinder, is connected with the condenser, while a passage is opened from the pipe m n to the steam box. Thus a communication is alternately made between the top and bottom of the piston.

The slide-valve represented above is not invariably employed in the double-acting engines, and we frequently find the annexed contrivance resorted to, in some of the best engines.

The pipe 14 represents the passage to the cylinder, and a communication is now opened with the steam chamber g. The raised valve is perforated and a similar valve beneath closed by the rod which passes through it. On closing the valve g, the lower valve h is opened, and a free passage between the condensing pipe beneath and the upper part of the cylinder is the result. If we now suppose a similar double valve placed at the bottom of the cylinder, it will easily be seen that an effect similar to that described in the sliding valve will be produced.

The speed of the engine is regulated by a very ingenious contrivance introduced by Mr. Watt, called the governor, and represented beneath.