THE STEAM-ENGINE. 



489 



Fier. 51. 



creasing in magnitude downward, to receive the piston-rod, in which the 

 latter is secured by a cross-pin B. A deep groove, square in its section, is 

 formed around the piston, so that while the top and bottom of the piston 

 form circles equal in magnitude to the section of the cylinder, the interme- 

 diate part of the body of the piston forms a circle less than the former by 

 the depth of the groove. Let a ring of brass, cast-iron, or cast-steel, be 

 made to correspond in magnitude and form with this groove, and let it be 

 divided as represented in fig. 51, into four segments C C C C, and four cor- 

 responding angular pieces D D D D. Let the groove which surrounds the 

 piston be filled by the four segments with the four wedge-like angular pieces 

 within them, and let the latter be urged against the former by eight spiral 

 springs, as represented in fig. 50 and fig. 51. These springs will abut 

 against the solid centre by the piston, and will urge the segments C against 

 the cylinder. The spiral springs which urge the wedges are confined in 

 their action by steel pins which pass through their centre, and by being 

 confined in cylindrical cavities worked into the wedges and into correspond- 

 ing parts of the solid centre of the piston, as the segments C wear, the springs 

 urge the wedges outward, and the points of the latter protruding, are gradu- 

 ally worn down so as to fill up the spaces left between the segments, and 

 thus to complete the outer surface of the piston. 



Various other forms of metallic pistons have been proposed, but as they do 

 not differ materially in principle from those we have just described, it will 

 not be necessary here to describe them. 



