PISTONS. 



Fig. 13. 



in fig. 12 and fig. 13. These springs will abut against the solid 

 centre of 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 corresponding parts of the solid centre of the piston, as 

 the segments c wear, the springs urge the wedges outwards, and 

 the points of the latter protruding, are gradually worn down so as 

 to fill up the spaces left between the segments, and thus to com- 

 plete the outer surface of the piston. 



21. The force with 

 which the piston is 

 moved from end to end 

 of the cylinder is esti- 

 mated by the pressure 

 of the steam which acts 

 upon it, diminished by 

 the reaction of the steam 

 escaping from the side 

 towards which it moves, 

 and the resistance pro- 

 duced by its friction 

 against the sides of the 

 cylinder. 



22. The mechanical 

 force with which the 

 piston is thus moved 



would be practically useless unless an expedient were provided by 

 which it could be transmitted to some convenient point outside 

 the cylinder, and since it is essential that the steam which impels 

 the piston shall be confined within the cylinder, and that no air 

 be allowed to enter, so as to react on the other side of the piston 

 by its pressure, it is also essential that whatever be the means of 

 transmitting the force of the piston to the outside of the cylinder, 

 it shall be accomplished without leaving any interstitial space 

 through which steam can escape or air enter. 



23. This object is perfectly attained by. a very simple con- 

 trivance. A hole is made through the centre of the piston, 

 in which a truly formed cylindrical iron rod, called the piston- 

 rod, is inserted and firmly fixed by a key or linch-pin. This 

 piston-rod passes through a hole made in the iron cover of the 

 cylinder, as shown in fig. 14. The piston-rod is kept in steam- 

 tight contact with the edges of the hole by a contrivance called 

 a stuffing box, B, represented in fig. 14. The hole made in the 



cover of the cylinder is very little 



greater in magnitude than 

 21 



