•218 
OF THE PARTS OF 
[sect. VII. 
into a recess in the cover of the cylinder. This recess is surmounted by a plate 
fixed on with screws, called a cap or bonnet, that can be easily taken off, or put on 
again in its place. 
The other method is similar in principle, but different in construction. Instead 
of having several screws all worked down by one motion, there is in this but one 
screw, and that one is a part of the piston rod, Plate vn. Fig. 2; on this is placed 
a wheel cl of a convenient diameter, the hole in the centre of which is a female 
screw, cut to work into that of the piston rod. The wheel is turned round so as 
to tighten the piston by means of a pinion a, provided with a square projecting 
head for that purpose; rising into a recess in the cylinder cover of the kind already 
described, and the cover or top plate is prevented from turning with the wheel by 
means of the pins e e, called steady pins. 
Metallic Pistons. 
469. Cartwright’s piston. The idea of employing metal instead of elastic 
vegetable matter, to render the pistons of steam engines tight, was one part of the 
patent obtained by Cartwright in 1797. 1 It consisted in using six or more solid 
masses of metal in the place of the usual packing; these masses being segments of 
rings, a a, Plate vn. Fig. 3. made to fit the internal surface of the cylinder, with a 
second series b b, crossing the joints of the other, and both series were pressed 
against each other and the cylinder by V-springs; and by having two sets, with 
the joinings of the rings in the one set, opposite the solid parts of the rings of the 
other set, the escape of steam at the joints was to be prevented. The upper and 
lower parts were connected by plates, to which the piston rod was joined. (See the 
section, Fig. 3.) 
The two exterior rings of brass were made of the full size of the cylinder, and cut 
into several segments, as shown at a a a, and laid one above the other so as to cross 
the joints. The joints in the under rings are shown by dotted lines in the figure ; 
and in like manner are disposed the two interior rings, both being confined to their 
1 Mr. Watt tried metallic packings in some of his early engines, but gave them up on account 
of the practical difficulties in keeping them tight, and from their wearing the cylinders unequally. 
They are now much employed in the cylinders of sea-going vessels, where opportunities for 
packing cannot be embraced without considerable inconvenience and loss of time; hut in point 
of effect, it is questionable whether there be any saving. 
The best description are those manufactured with one or more cast iron or brass rings, the joints 
of which overlap, with a packing behind : this is preferable to those constructed with springs, 
which soon lose their elasticity and become useless.