218 OF THE PARTS OF [SECT. vn. 



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 d 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. ' 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 vu. 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 



i 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 ; but 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. 



