LOCOMOTIVE ENGINE. 431 



These arms and the centre boss stand up 2^ inches from the plate A, and another 

 plate, D, of the same thickness as the first, is put upon them, making the whole thick- 

 ness of the piston 3f inches. The plate D is held steady in its place by a projection 

 on the centre boss passing through it, and is fixed by screws CC, tapped into the 

 bosses at the ends of the arms C, the heads of the screws CG being countersunk 

 into the plate D. The plates A and D are turned so as to be just capable of 

 moving in the cylinder without touching it, and three brass rings FF are placed 

 between them. The inner ring is three-eighths of an inch thick, and is the same 

 width as the space between the plates ; the two outer rings are half an inch thick, 

 and of half the width ; and one of them has a projecting ring or rebate upon its 

 edge, fitting into a corresponding groove in the other to keep them steady. The 

 rings are turned exactly to fit the cylinder and each other, and cut through in one 

 part, having been first hammered a little all round on the inside, which gives them 

 a tendency to expand, and causes them to fly open on being cut ; when, therefore, 

 they are put in their places in the cylinder, they press against the cylinder by their 

 elasticity, and keep in close contact with it, so as to make a steam-tight joint during 

 the motion of the piston. The divisions in the rings are placed in opposite positions, 

 or break joint, in order to prevent the escape of steam through them ; for if they 

 were to coincide, a passage would be left for the steam through the piston. 



The elasticity of these rings is found to be quite sufficient to keep the piston steam- 

 tight when moved in the cylinder, and it continues so for a long time ; when, however, 

 the rings become so much worn by the friction as to have expanded nearly to the ut- 

 most, some other means is necessary to press them against the cylinder. For this pur- 

 pose the three steel springs OGG are placed in the piston; they are of the same 

 width as the inside ring, against which they bear, and one-eighth of an inch thick in 

 the middle, and a pin is put through each of them, having a collar bearing against 

 the spring, and screwed at the other end into the centre boss of the piston ; by un- 

 screwing the pin a little the spring can be made to press harder against the ring 

 when required, and the pin is then fixed by screwing up the set-nut upon it against 

 the boss. When the piston is first made and the rings are new, these springs are 

 not required, and they are set so as only to touch the rings ; but as the rings wear 

 and become too loose in the cylinder, the springs are screwed up more and more, 

 and made to press harder against the rings ; and when they are very much worn 

 they are kept tight to the cylinder by the springs only, as they have expanded to 

 their utmost. Access is readily obtained, when necessary, to the inside of the 

 piston, by taking off the front cylinder cover and unscrewing the front plate D of 

 the piston. 



The pistons are often made upon different modifications of Barton's prin- 



