220 OF THE PARTS OF [SECT. vn. 



segments, acted on by eight spiral springs of tempered steel. These springs are 

 inserted in cylindrical cavities at both ends, in order to render them secure from 

 bending, and yet allow them to play freely. With the same view, each spring has 

 a cylindrical pin of steel within it, a little shorter than the spring. In pistons 

 made for high pressure steam there are three grooves, formed round the exterior 

 part of the segments, as in Fig. 5. ; the middle one a designed to hold oil or grease, 

 to lubricate the rubbing surfaces. The upper and lower grooves, b b, are for hoops 

 of tempered steel, having a forked loose joint, as Fig. 6., at one point in each. 

 These hoops are nicely fitted to the grooves ; and when the piston is placed in the 

 cylinders their jointed ends meet. Each hoop is prevented from turning round in 

 the groove by a pin or stud, in order that the two hoops may not have their joints 

 opposite to each other. These hoops, or rather springs, form an important addi- 

 tion, and assist greatly in preventing the leakage, which otherwise would take place 

 through the unequal expansion of the segments and wedges ; for the point of the 

 wedge will move outwards over n m, Fig. 4., while the segments move only over 

 n o, and consequently would wear the cylinder into grooves, were it not rounded 

 off, and the hoops added, to prevent the escape of steam. 



But by combining hardness and elasticity, Barton has done much to render these 

 pistons tight and durable : they still however depend chiefly on the skill of the 

 workman : when they are done well by a person who understands them, they 

 undoubtedly answer effectively, so long as the springs retain their elasticity. 



471. To avoid the effect which the unequal expansion of the parts of Barton's 

 piston produces, I would recommend the construction shown by Fig. 7., where the 

 wedge-formed pieces do not extend to the surface of the cylinder ; and to prevent 

 there being an aperture at each joint, two series of segments and wedges should be 

 used, as shown in the section : the joints of the lower series are shown by dotted 

 lines in the plan. 



472. It is of importance to remark, that the metallic packing is pressed so as 

 to be steam-tight by the steam itself; and it is essential to their perfect operation 

 that the steam has egress to the cavities in the piston, and that the parts fit 

 perfectly against each other in all the horizontal joints. Let strong steam be on 

 the upper side A, of the piston, Fig. 7. and the lower side B, be open to the con- 

 denser ; then the steam enters at the joints e e, presses the segments close on the 

 lower plates, and fills the interior so as with the assistance of the springs to press 

 the segments outward against the cylinder. Also, when the lower side is open to 

 the steam, and the upper one to the condenser, the steam enters at //, pressing the 

 segments close against the cylinder and upper plate. If this were not so, the 

 springs could not possibly press with sufficient force to keep the joints steam-tight ; 



