216 
OF THE PARTS OF 
[sect. VII. 
466. The atmospheric engine piston consists of a plate of cast iron, about 
one-eighth of an inch less in diameter than the cylinder, and about one inch and 
a half thick, formed with a rim about four inches from the edge. A flat ring 
corresponding to the part beyond the rim is fitted upon it, and both have holes 
for bolts to screw them together, which is done after a packing of soft hemp or 
gasket, saturated with tallow, has been inserted. In order to render it more tight, 
a portion of water is kept constantly on the upper side of the piston. 
Smeaton had a superior method of constructing the piston for atmospheric engines, 
which rendered the loss by the condensation of steam much less. The construction 
of that for the Chase Water engine, with a seventy-two inch cylinder, being given, 
will show this method. The bottom of the piston was made of wooden planks 
fastened by bolts to the piston plate, with rings on the under side of the planks 
to receive the heads of the bolts. The advantage of wood for this purpose, in cases 
where the injection is made in the cylinder, is obvious. See Fig. 1. Plate vi. 
The plank bottom, of elm or beech, was about two inches and a quarter thick 
when worked, and was formed by two planks halved together, in the form of a 
cross, and grooved on the edges with a three-quarter inch groove, to receive the 
ends of the pieces to fill the corners between the cross, put in so that the pieces 
may have the grain radiating from the centre : a few rivets to hold the cross 
planks together were inserted, where they were halved into each other, at their 
intersection, and the whole being hooped with a good iron hoop half an inch thick 
and two and a quarter broad, it bound all tight together. The outside diameter of 
the hoop was a quarter of an inch less than the cylinder. The flat iron rings for the 
under surface of the piston should be let in flush with the surface of the wood, 
and the bolt heads counter-sunk: the planking was screwed on with a double 
thickness of flannel and tar between it and the iron piston plate, and any irregular 
hollows filled up with additional thicknesses of flannel and tar, so as to exclude the 
air between the plate and the wood. The bolts were carefully secured so as to 
make a water-tight joint from above. The plank was covered on the lower side 
by a lining of deal boards, shot clear of sap, and three quarters of an inch thick, 
nailed to the planks, with a single thickness of flannel and tar between, so as to 
exclude the air; after this the lower surface of the lining was made perfectly flat 
and smooth. 
467. The hemp packed piston is now most commonly employed for steam 
engines, and the usual mode of construction is as follows : the bottom of the 
piston b (Plate vii. Fig. 1.) is fitted as accurately to the cylinder as it can be done, 
to leave it at full liberty to rise and fall through the whole length. The part of 
the piston immediately above this is from one to two inches, according to the 
size of the engine, less all round than the cylinder, to leave a circular space into