SECT, vii.] STEAM ENGINES. 221 



for a fluid cannot be confined by a force less than its own elastic force ; and hence 

 the pressure producing friction is always greater than the pressure of the steam on 

 the rubbing surface, by that due to the pressure of the springs. 1 



473. JESSOP'S PISTON. A completely different method of applying metal to 

 render pistons steam-tight was invented by Mr. Jessop, and secured by patent in 

 1823. It consists of an expanding coil of metal, which binds round the piston 

 body in a spiral form. Fig. 8. Plate vii. shows a section of a piston of this kind, 

 where A A is the elastic spiral of metal, which, when at liberty and removed from 

 the piston, assumes the form shown in Fig. 9. To form a piston of this kind, a 

 bed of hemp packing, B B, is first prepared, which answers the double purpose of 

 preventing steam passing at the joints, and of supplying a means of pressing the 

 springs against the surface of the cylinder. A small addition of hemp packing is 

 at times necessary to make up for the wear. 



The action of the steam in keeping this piston tight, is by pressure on the top 

 and bottom plates, as in the common hemp-packed pistons. The pressure and 

 wear of these pistons will be more equable than in the other metallic kinds when 

 they are equally well made, and they have been as successful in practice. 2 



474. OF THE FRICTION OF PISTONS. The rubbing surface of a piston must be 

 pressed against the cylinder with a force at least equal to the pressure of the steam 

 it confines, otherwise the surfaces would separate and the steam escape. Now it 

 has been shown (art. 463.) that the thickness of the rubbing surface should be 

 equal to that portion of the diameter which expresses the friction ; therefore, let r 

 be the friction when the pressure is unity, t = the thickness, a = the diameter, and 

 p = pressure of the steam ; then, IT t a p r = the friction ; or since t = r a, it is 

 Tf pa- r 2 = the friction, to which one-tenth may be added for that of the piston rod. 



The moving force is, 



ir pa? 



consequently that part of the moving force equal to the friction is, 



K p tft 



1 So little is known by many mechanicians of the nature of the action of pistons, that it is not 

 unusual to hear them express an opinion on the friction of a piston from the force required to 

 move it in an open cylinder ; and on a level with it is the method of estimating the friction of an 

 engine by the power it requires to move it when it is doing no work. The true state of the fact is, 

 that the friction is as the stress on the parts, and this stress bears a relation nearly in proportion to 

 the work done. 



z An arrangement of the parts of a metallic piston was one of the objects of a patent obtained 

 by Perkins ; but as it is inferior to those already described, it will be sufficient to refer to it. 

 See Repertory of Patents, vol. i. p. 224. 



