SECT. VII.] 



STEAM ENGINES. 



22:3 



packing, and cause it to press against the rod ; it is cup-formed at the top to 

 contain tallow to grease the rod. See Plates iv. and v. 



477. Metallic packing was tried for piston rods by Cartwright, and has since 

 been much improved by Barton : it is however a part of so much less importance 

 than the piston, that it will not be very often thought prudent to be at the expense, 

 though the ingenuity of the contrivance renders it desirable to describe it. Bar- 

 ton's metallic substitute for stuffing boxes is shown in the annexed figures ; where 

 D is the piston rod, E the box with a ledge for the cast iron plate F to rest on, and 

 G another above it to receive the cast iron plate H. The cover I of the box is 

 secured by screws in the usual manner, with plates of lead in the joints J and K, 



FIG. 21. 



for the purpose of making the joints closer. The three principal metal blocks L 

 embrace the piston rod D, and three wedging blocks M fill up the spaces between 

 them. Two thin hoops N N of tempered steel, firmly riveted together at their 

 ends, surround the outside of the blocks, binding upon the rounded exterior angles 

 of the blocks, and these angles are left on in the middle to keep the hoops in their 

 places. At each of the exterior angles of the blocks L, there are two spiral springs, 

 fitted to cylindrical holes, and also provided with cylindrical pins, as those of the 

 piston. By these and the elastic hoops the blocks L are strongly pressed towards 

 the piston rod. Two other hoops a a of elastic steel, cut across, are inserted in two 

 grooves to be in contact with the rod, and serve to close the joints more perfectly : 

 they are fixed in a similar manner to the rings round the piston before described. 



