432 
STEPHENSONS 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times in stationary engines is now generally superseded by metallic packing, as 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-J- 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^ 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 ri, 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 oi 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 o', into the blast pipe, rushing