298 APPLIED SCIENCE 



present we may neglect these considerations, and assume that 

 we know the stress in the driving or resisting link. 1 We then 

 have the frame shown in Fig. 15a. 



If the same engine, Fig. 16, were employed to overcome or 

 transmit a couple substituted for link 6 or element /, the 

 dynamic frame would become that shown in Fig. 16a. The 

 engine is shown with the piston rod in tension ; links 2 and 5 

 must be first drawn tangent to their friction circles, then the 

 bearing pressures parallel respectively to 2 and 5, and tangent 

 to the friction circles for eb and ec. The line of pull in the 

 element e is given by the line joining the intersection of the 

 bearing pressures with that of 2 and 5 ; the diagram is completed 

 by the bar perpendicular to 5 and that perpendicular to 2 ; 

 these bars are mere indications of the arms of the equal and 

 opposite couples exerted on elements c and b. These elements 

 must be stiff, and their rotation relatively to one another is, by 

 hypothesis, resisted by a couple such as would be produced by 

 friction exerted on a wheel forming part of b revolving between 

 clips or rubbers forming part of c. The diagram supposes that 

 the frictional resistance thus obtained is so exactly equal at 

 opposite ends of a diameter of the friction wheel as to constitute 

 a couple which does not directly affect the pressures on the 

 joints eb, ec. 



18. Ordinary Direct-acting Steam-Engine. The ordinary 

 direct-acting steam-engine with a single cylinder gives another 

 example of a simple complete machine. Fig. 17 shows a sketch 

 of an engine of this type with the resistance exerted as if by a 

 link between the periphery of a fly-wheel and the fixed element 

 or bed-plate. We will assume that the position and direction 

 of this resistance is known, being that shown by the arrow on / 

 in Fig. 17. The elements are: a, the piston rod and block 

 sliding on the guide bars ; fc, the connecting rod ; c, the crank, 

 axle, and fly-wheel ; d, the bed-plate, including the cylinder ; e, 

 the steam in the cylinder. This element is jointed with a and 



1 The steam, piston, and cylinder constitute, with the resisting link, a 

 simple inclined plane machine, vide 15, and this machine drives the second 

 machine, constituted by the piston rod, connecting rod, crank, bed plate, and 

 resisting link ; the piston rod and bed plate are common to the two machines, 

 ride 24, 25. 



