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.