SECT. VII.] 
STEAM ENGINES. 
217 
which unspun long hemp, or soft rope prepared for the purpose, and called gasket, 
is wound as evenly and compactly as possible, to form the packing. This packing 
is compressed together by a plate or cover C, which is put over the top of the 
piston, having a projecting ring to fit over the lower part, and complete the upper 
side of the space for the packing, the pressure being produced by screws S S, &c. 
Both the upper and lower part of the space round the piston, to contain the 
packing, is a little curved, that the pressure produced by the screws on the 
packing may force it against the inside surface of the cylinder, into as close 
contact as possible. 
The screws being tightened when the piston is in the cylinder, the particular 
form of the piston has the effect of squeezing out the packing, and causing it to 
press forcibly against the inside of the cylinder at its upper and lower edges. 
When the packing wears so as to become too small by use, these screws, which 
are more or less in number according to the size of the piston, are always resorted 
to for tightening it, as long as they are capable of acting; and when this is no 
longer the case, the piston top must be removed, and an additional quantity of 
new packing introduced. The piston rod is generally attached to the bottom part 
of the piston, by passing it upwards into a conical hole made to receive it, to 
which the bottom of the rod is exactly fitted, and a screw nut, or a wedge, between 
the top and bottom is inserted, which effectually secures it. 
The piston is kept supplied with melted tallow by means of a funnel on the 
top of the cylinder lid, provided with a cock to prevent the escape of steam. 
468. Woolf’s piston. In the usual method, whenever the piston, by continued 
working, becomes too small and occasions a waste of steam, it is necessary to take 
off the top of the cylinder, in order to get at the screws, even when fresh packing 
is not wanted. This being laborious work, is therefore generally avoided by the 
person who attends the engine, as long as it can possibly be made to work without 
taking this trouble ; and the neglect occasions a great and unnecessary waste of 
steam, and consequently of fuel in proportion. 
The object of Mr. Woolfs improvement is to enable the engine man to tighten 
the piston, without the necessity of taking off the cover of the cylinder, except 
when new packing becomes necessary. He accomplishes this by the following 
methods. 
To the head of each of the screws a small toothed wheel is fixed, so that it may 
be turned, and therefore tightened, by means of a central toothed wheel, which 
works upon the piston rod as an axis : if one of the small wheels be turned, it 
turns the central wheel, and the latter turns the others. The one which is to be 
turned by the handle is furnished with a projecting square head, which rises up 
2 E