334 EXPLANATION OF THE PLATES. 



PLATE VII. 



This plate is to represent the construction of pistons. 



Fig. 1. represents a section, and part of the plan, of the common packed piston, 

 which is tightened by the screws S when it wears. See art. 467. 



Fig. 2. shows one of Woolfs methods of tightening the whole of the screws at 

 once, and consequently in the most regular manner, without having to take off the 

 cylinder lid. See art. 468. 



Fig. 3. is the plan and section of Cartwright's metallic piston, (art. 43.) as at 

 first executed ; it was afterwards made with spiral and other springs, acting only 

 on the interior segments of rings b b, but it required that the outer ones should be 

 moved ; and this has been done by making the interior segments in short pieces to 

 cover the joints of the outer ones, so as to leave space to insert springs between 

 them, to act directly on the outer segments, by Mr. Lloyd : it has also been done 

 by inserting a short cylindrical piece against each joint of the outer ring, both the 

 cylindrical pieces and the outer segments being pressed outwards by spiral springs : 

 this method is employed by Messrs. Hall. See art. 469. 



Fig. 4. represents Barton's construction of metallic pistons in plan and section. 

 The points of the wedges G expand faster than the segments E in the ratio of m n 

 to o n, and hence wear the cylinder unequally. To prevent this, the points are 

 shortened, and two elastic hoops b b, Fig. 5. are put round, neatly fitted with a 

 loose tongue joint, as shown at Fig. 6. See art. 470. 



Fig. 7. shows a method of avoiding the defect of Barton's piston, by keeping the 

 points of the wedges as much within the segments of the piston as may be necessary 

 for wear, before a change of parts becomes necessary, and making two series to 

 break joints. See art. 471, 472. 



Fig. 8. represents a section of Jessop's piston, and Fig. 9. the expanding coil of 

 metal which rubs against the cylinder. See art. 473. I am of opinion that a 

 more perfect method of pressing the packing against the metal coil, is the only 

 thing wanted to render this decidedly the best kind of piston. 



The friction of pistons is investigated in art. 474. 



