Pistons packed with hemp and tallow continued in use, with very little variation, ffom the time of Captain Savery to that of Dr. Cartwright, a period of ninety-ninepars. That scientific clergyman had, however, the honour of first introducing, in an ingenious engine of his own construction, an expanding or elastic piston made entirely of meta4— an invention of indispensable utility in all engines working at high pressure.
As this piston has been considerably modified by various engineers, which we shall have to notice hereafter, we shall here state briefly that It consisted of two rings of brass, of the full size of the cylinder ; these rings were each cut into three or more segments, and laid one over the other, so as to break the con tinuity of the vertical joints between the segments ; concentric with those seg ments were a similar arrangement of segments inside the others, which were intended to stop the steam from passing horizontally; in the cavity between all the segments were placed feather springs, designed to press the segments outward to fill up the cylinder as either that or the piston wore. These pistons, however, proved very defective ; for as the exterior segments wore, and conformed to a larger circle, the inner segments, which had no wear, were no longer concentric with those outside of them, and crevices were thus opened, through which the steam escaped past the piston.
Amongst those persons who directed their attention to the improvement of this important part of the engine, Mr. John Barton was the most successful. In the annexed engravings, lig. I represents a plan of the piston, with the top plate removed, and Fig. 2 a vertical section of the same, taken on the line 1, e d on the plan. a a a a are four metal segments ; WI' four right-angled wedges interposed between the segments, their points forming a pm-dm of the periphery; c c c c is a thin steel spring, formed into a single broad hoop, and pressed into the undulated form represented, by which it is found to act with uniform energy upon the wedges, until they and the segments become so much worn in the course of time, that the steel spring recovers itself into its original circular figure ; d is the frame-work cast in one piece with the lower plate of the piston ; e is the piston rod ; the dark spaces shown on the plan within the circular frame d are cavities to lessen the weight of metal; the other dark spaces are cavities to allow of the free action of the circular spring.
To prevent the segments from falling out of their places whilst the piston is being taken out, or put into the cylinder, the periphery of it is grooved near to its upper and lower edge, in which are sunk two slight spring hoop., cleft across into forked joints, which close together simply by their elasticity. To lubri cate the piston, there is a third groove, made midway between the two former for the reception of the oil ; these parts are not introduced into the figures. The action is as follows : as the piston and cylinder wear away by the friction, the circular spring c presses out the wedges 6, and these project the segments against the cylinder; and as the segments become reduced, the wedges fill up the increasing opening between them.
An objection has been raised against this piston, that as the wedges must move through a greater space than the segments, in order to press the latter into the circumferential the wedges must in consequence rub twice as much against the cylinder, and consequently score it. This objection we believe to be unfounded; and as far as our experience and obrirvation have extended, we have found the wear very uniform. Mr. Barton, perhaps, softens the segments, or makes them of an alloy, which is more easily abraded than the segments; sometimes (we have been informed,) he obviates the supposed tendency of scoring, by cutting out a portion of the end of the wedges, so that they do not bear upon their whole depth or thickness against the cylinder, consequently they will abrade twice as fast as the segments, supposing them to be equally hard. A great variety of metallic pistons have been made of late years, but we know of none that have so !idly answered the purpose as the recently patented im provement by Mr. John M'Dowall, of Johnston, near Paisley, who has a manufactory of them at Manchester, where, we understand, great numbers are advantageously working in the engines of the factories. We have seen them in other parts of the kingdom, and can attest their superior excellence.