214 
OF THE PARTS OF 
[sect. VII. 
Pistons may be rendered tight by an elastic packing of vegetable or animal 
matter; but the latter kind of packing cannot be used for steam, on account of 
the heat destroying it. 
Pistons may also be made wholly of metal, constructed so as to admit of a 
certain degree of elasticity. 
After considering some particulars common to all pistons, we will treat of 
pistons, as below, dividing them into two classes. 
on wood, 
on metal. 
f Common leathered 
Packed <( Atmospheric engine piston. 
Common. 
Woolf’s. 
Hemp 
Pistons <( 
f Cartwright’s 1797. 
J Barton’s 1816. 
| Jessop’s 1823. 
Perkins’ 1823. 
463. When a piston rod is to be pushed as well as drawn, unless it be of a 
certain thickness in proportion to the diameter, it is liable to stick if there be the 
slightest inequality in the friction, or in the centring of the rod. If it were a thin 
plate, nothing but its connexion to the rod would prevent it turning with the 
slightest inequality of its friction, on being pushed; and as we make it thicker, 
the thickness interferes more and more with any tendency to turn. The propor 
tions which will secure us from the risk of this evil are not difficult to ascertain. 
Let the pressure on the piston A B move the rod C D. 
Then, in order that the piston may move steadily, its friction 
at the circumference multiplied by half the diameter of 
the piston, should be equivalent to the pressure producing 
that friction, multiplied by half the thickness of the piston ; 
consequently, the thickness should be to the diameter, as the 
friction is to the pressure of the rubbing surfaces. 
Fig. 18. 
c 
¿s 
The friction of brass on iron is at an average one-eighth 
of the pressure; hence the thickness of metallic pistons 
should not be less than one-eighth of the diameter. 
The friction of hemp packing on iron is about one-sixth of the pressure, hence 
the thickness of the packing should be one-sixth of the diameter. Practice is