Catch-pins, _x_, are provided, to prevent the beam descending too far should the engine make too long a stroke; two wooden springs, _y y_, receive the blow. The great beam is carried on sectors, _z z_, to diminish losses by friction.
The boilers of Newcomen's earlier engines were made of copper where in contact with the products of combustion, and their upper parts were of lead. Subsequently, sheet-iron was substituted. The steam-space in the boiler was made of 8 or 10 times the capacity of the cylinder of the engine. Even in Smeaton's time, a chimney-damper was not used, and the supply of steam was consequently very variable. In the earlier engines, the cylinder was placed on the boiler; afterward, they were placed separately, and supported on a foundation of masonry. The injection or "jack-head" cistern was placed from 12 to 30 feet above the engine, the velocity due the greater altitude being found to give the most perfect distribution of the water and the promptest condensation.
[Illustration: FIG. 22.--Boiler of Newcomen's Engine, 1768.]
Smeaton covered the lower side of his steam-pistons with wooden plank about 2-1/4 inches thick, in order that it should absorb and waste less heat than when the iron was directly exposed to the steam. Mr. Beighton was the first to use the water of condensation for feeding the boiler, taking it directly from the eduction-pipe, or the "hot-well." Where only a sufficient amount of pure water could be obtained for feeding the boiler, and the injection-water was "hard," Mr. Smeaton applied a heater, immersed in the hot-well, through which the feed passed, absorbing heat from the water of condensation _en route_ to the boiler. Farey first proposed the use of the "coil-heater"--a pipe, or "worm," which, forming a part of the feed-pipe, was set in the hot-well.
As early as 1743, the metal used for the cylinders was cast-iron. The earlier engines had been fitted with brass cylinders. Desaguliers recommended the iron cylinders, as being smoother, thinner, and as having less capacity for heat than those of brass.
In a very few years after the invention of Newcomen's engine it had been introduced into nearly all large mines in Great Britain; and many new mines, which could not have been worked at all previously, were opened, when it was found that the new machine could be relied upon to raise the large quantities of water to be handled. The first engine in Scotland was erected in 1720 at Elphinstone, in Stirlingshire. One was put up in Hungary in 1723.