SECT. VII.]
STEAM ENGINES.
219
places by a top and bottom plate, to which the piston rod is fixed. The segments
are pushed away from the centre by steel springs, of the form of the letter V.
Pistons on Cartwright’s plan have not been quite successful in practice, when
the cylinders have not been truly bored; and the causes were pointed out very
clearly by Mr. W. Nicholson, soon after the invention was brought before the
public. 1 The pieces forming the piston having a determinate curvature, and being
too strong to be sensibly flexible, cannot be expected to accommodate themselves to
any irregularity in the cylinder in different parts of its length, as is done by the
elastic stuffing of hemp ; and there is reason to doubt, in applying them, whether
the pressure of the rings or pieces together has not been too powerful for the
springs to perform their office when applied in this manner.
As to the actual difference between the friction of metal, and hemp against
metal, when the pistons are equally steam-tight, it is undoubtedly in favour of
metallic pistons (art. 463).
470. Barton’s piston. A piston considered superior to Cartwright’s was made
by Mr. Barton, Plate vn. Fig. 4. It consists of one thick ring E, of brass or cast
iron, made very nearly to fit the cylinder, and then cut into three or more equal
segments : the equal triangles remaining are used as wedges to expand the seg
ments of rings into a larger circle. The segments, and small triangles or wedges,
are secured between a top and bottom plate, as in the piston last described, with
spiral springs to press the triangles outwards from the piston rod, making them act
as wedges to press the segments against the inside of the cylinder; and as these
wear by use, the points of the wedges themselves protrude, and, being formed of the
same metal, still make part of the piston. A piston of this kind, and a true
cylinder, has been known to work for some years without requiring any other
attention than keeping it properly greased ; but it is easy to prove that the wedges
and segments do not expand equally, hence in this state it was not applicable to
high pressures ; besides, the imperfection of Cartwright’s piston still remained. It
has however been recently much improved by Barton, and therefore I propose to
describe it more fully in its improved state.
The piston is represented by a plan and section, Fig. 4. It is composed of a
solid cylindrical cast iron body A, having a conical hole B, to receive the enlarged
end of the piston rod, to which it is secured by a cross pin D, passing through
both. A space or groove is formed round the body of the piston, to receive four
brass, cast iron, or cast steel, hardened and tempered segments marked E, which
are spread asunder by four triangular wedges G, of the same metal as the
1 Philosophical Journal.