432 STEPHENSON'S PATENT 



ciple ; with two or more rings of cast iron or brass, about an inch thick, cut into 

 three or four segments, and having wedges inserted between them, which are con- 

 stantly pressed outwards by springs, so as to keep the segments always tight to 

 the cylinder ; the springs are either spiral or flat springs like those in the engrav- 

 ing, or a circular steel hoop, a little larger than a circle touching the ends of the 

 wedges, is forced in so as to bear against them. These pistons are liable to a de- 

 fect from which the other with the spring rings is free ; that of wearing grooves in 

 the cylinder where;! the points of the wedges rub against it, as the wedges have to 

 wear down faster than the segments ; the plan shown in the engravings is very 

 efficient, and appears on the whole to be the best. The hemp packing used some- 

 times in stationary engines is now generally superseded by metallic packing, as 

 it requires frequent renewal, and is unequal in its pressure, the piston having to be 

 packed very tight at first, in order to keep tight for any considerable time ; in 

 a locomotive, where the motion of the piston is very rapid, it would be quite in- 

 admissible. 



H H, or Y, (Plates XC. and XCI.,) is the piston-rod ; it is If inches diameter, 

 and is made conical at the end, being increased to 2-| inches diameter in the centre 

 boss of the piston, which is fitted upon it very exactly, and fixed by the cotter or 

 key I, half an inch thick, and tapered slightly from 1-J inches wide; the piston rod 

 has to be fixed very firmly, and is split at the end, to prevent its getting loose. 

 The other end of the piston rod passes through the stuffing box K, (figs. 15 and 16,) 

 in the cylinder cover ; it is made of steel, and turned truly cylindrical, to move 

 through the stuffing box with as little friction as possible. The stuffing box 

 has a half inch space round the piston rod for the packing, which rests against the 

 brass ring or bush L, fitted on to a small flanch at the end of the stuffing box, 

 and is compressed by the brass gland M, which leaves about three inches for the 

 packing. The gland has two projections on the outer end, making an oval shape, 

 and is held by bolts passing through these projections and screwed into correspond- 

 ing projections on the stuffing box. 



The piston is impelled by moving the slide valve from its central position, so as to 

 admit the steam from the steam-chest into the cylinder through one of the ports, as 

 in Plate XC., where the front port n is shewn open ; and the steam pressing against 

 the front of the piston impels it to the back end of the cylinder. The slide is then 

 moved to the opposite position, covering over the front port n, and opening the back 

 port m, to admit the steam behind the piston and impel it back again to the fore 

 end of the cylinder ; at the same time allowing the steam in front of the piston, 

 which had impelled it before to the back of the cylinder, and is now waste steam, 

 to escape by the inside of the slide and the waste port d, into the blast pipe, rushing 



