APPLICATION OF GRAPHIC METHODS 285 



relatively to its eye when e contracts, and / is lengthened ; 

 before doing this, we must choose in which element the pin is 

 to be fixed. The links of the dynamic frame are then drawn 

 tangent to the friction circles, so as to make the stated angle 

 with the surface of each joint. The point where the pin bears 

 against the eye, marked by a dot on the surface of each pin, 

 will aid in showing the meaning of the diagram, as explained in 

 7. The italic letters near the arrows denote the element in 

 which the pin is fast, and the arrows show the direction of 

 rotation of that element relatively to the other element of the 

 joint. If/ were made the driver the direction of these rotations 

 would all be reversed, and consequently the links would all have 

 to cross over to the other sides of the friction circles. This 

 would materially alter the form of the diagram, and in many 

 machines would result in an impracticable diagram, showing 

 that the machine cannot be driven backwards. 



9. Modification of the Dynamic Frame Couples. When any 

 element of a machine is in equilibrium under the forces applied 

 at two joints the dynamic link will in direction and position 

 correspond more or less closely with the direction in which the 

 element of the machine lies between the joints. Thus, when 

 the machine constitutes a material frame in one plane, as in 

 Fig. 6, p. 281 the dynamic frame, Fig. 6a, will have a general 

 resemblance to the machine. The similarity between the actual 

 frame composed of material links and the ideal dynamic frame 

 in this case must not lead the reader to expect that he will 

 always be able to identify a link in the dynamic frame as cor- 

 responding to an element in the actual machine. Half a dyna- 

 mic link corresponds to each joint of the machine, and there 

 may be dynamic links which, like link 3 of Fig. 5, have no cor- 

 responding joint in the machine, but the dynamic link will only 

 correspond with an actual element when the number of joints 

 in the latter is limited to two. Since a two-jointed element has 

 a link resembling it, we may sometimes name this link by the 

 letter of the element ; thus, all the links of Qa might be called 

 indifferently by the letter of the element or the number assigned 

 to the link ; but in Fig. 56 or oc, we can name only the link 5 by 

 the letter of an element. In fact, the capital letter alwavs signifies 



