222 OF THE PARTS OF [SECT. vn. 



4'4 

 In double engines with metallic pistons, 4'4 r 2 = gg = '069 of the power. 



4.4 

 In double engines with hemp-packed pistons 4'4 r c = QQ = '1222 of the power. 



1 Tfi 



In single engines with hemp-packed pistons - 4 x 4'4 r z = ^ ^ = '049 of the 



power. 



In high pressure engines the friction is supposed to be in the same ratio ; but 

 the loss of steam past the piston being in respect to the power in the inverse ratio of 

 the diameter of the piston, I have assumed the friction and loss to be two-tenths 

 of the power, and that because it corresponded with observation in two cases where 

 I had a tolerably certain means of comparing the power and effect. Calculation 

 gives the loss a little more. See note to art. 384. 



PISTON ROD COLLARS, OR STUFFING BOXES. 



475. The piston rod collar, or stuffing box, is a contrivance for rendering the 

 place where a smooth rod or plunger passes into a vessel air-tight. This mode of 

 giving motion without admitting air must have been long in use ; we meet with it 

 in various works without an allusion to the time of its invention. It is so similar 

 to the construction of a piston, that a separate detail seems scarcely to be necessary. 

 As in the piston, so in this the effect is produced by elasticity ; and leather, hemp, 

 cotton, cork, and metal, have been used for the purpose. 



Where the heat of steam is not likely to be injurious, leather is generally 

 employed. It was first employed in discs, cut to fit the rod, and pressed together 

 by screws. The next was cupped leathers ; and the first instance of their applica- 

 tion seems to have been at the York Buildings water works, 1 and they were used 

 by Smeaton for his air pump : he also applied them to the piston rods of the 

 blowing machines at Carron, and describes how to form the cups by stamping 

 them into a cylinder of the size of the rod they are intended for. 2 What renders 

 Smeaton's stuffing box for the blowing cylinders more curious, is, that he uses a 

 block of hard wood for the rod to pass through, and the rods it seems were draw- 

 filed. The application of cupped leathers to the plungers of the hydrostatic press 

 by Mr. Bramah, put them to the test on a large scale, under immense pressures. 



476. The stuffing box with the hemp packing is made to fit tight round the 

 piston rod in a manner nearly similar to the piston. A collar with a hole through 

 it, just sufficient to give easy passage to the rod, is screwed down, to confine the 



1 Architecture Hydraulique, vol. ii. p. 62. Description of the Pumps of York Buildings Water 

 Works, London. 



4 Philosophical Transactions, vol. xlvii. p. 415. Reports, vol. i. p. 360. 



