216 OF THE PARTS OF [SECT. vn. 



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 vn. 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 



