APPLICATION OF GRAPHIC METHODS 299 



'I : the position and direction of the force exerted are in this 

 case determinate ; for the cylinder does not oscillate and the 

 piston with its rod are subject to no stress that is not axial ; the 

 element a is in equilibrium under the force due to e, and those 

 due to the joints ab and ad. In Fig. 17a link 1 is drawn coin- 

 ciding with the axis of the cylinder, and represents the bearing 

 pressure produced by e on its joints ; the link 2 may next be 

 drawn tangent to the friction circles for ab and be. The third 

 force under which a is in equilibrium is that due to the joint 

 ad ; the link 5 is drawn through the dynamic joint A, making 

 the stated angle with the guide bars. This diagram corre- 

 sponds to an engine in which the slide block is as usual fast on 

 the piston rod. 1 



We next observe that element c has three joints first at </, 

 secondly at be, and lastly at cd. The resisting link must, in 

 order that the machine may be complete, abut at its other end 

 against the bed-plate d ; it may be due to actual friction, as 

 when the fly-wheel is, for experimental purposes, fastened be- 

 tween two friction blocks secured to d. We know, by hypo- 

 thesis, the place and direction of its application, and therefore 

 the position and direction of link 6. The intersection of 6 and 2 

 gives the dynamic joint C ; the third force acting on c must 

 pass through this joint, and make the stated angle at the joint 

 cd ; we therefore draw link 3 from C tangent to the friction 

 circle for cd. Lastly, we observe that the element d is in equi- 

 librium under the following forces: 1st, that due to the joint ed, 

 equal and opposite to that on ae; 2nd, that due to the joint ad\ 

 3rd, that due to the resisting link/; 4th, that due to the joint, 

 cd. We have already, on the Fig. 17 a. the position and direction 

 of all these forces. They do not, however, all meet in one joint, 

 and we must, therefore, to complete the dynamic frame, add a 

 link which shall receive the equal and opposite resultants of the 

 forces compounded in two pairs ; this we may do by joining the 

 intersection between links 5 and 6 with that between 1 and 3, 



1 If, however, there were a joint between these parts, snch that the pressure 

 from the guide bars must pass very near the centre of the pin at that joint, 

 then links 5 and 2 would be first drawn, and link 1 drawn cutting their 

 dynamic joint ; this arrangement would cause the effort exerted by the piston 

 to pass a little way from the axis of the cylinder, as shown in Fig. lib. 



