278 APPLIED SCIENCE 



as represented by an ideal elastic link exerting definite, equal, 

 and opposite forces in a definite position and direction. This 

 ideal resisting link is equally suited to express the resistance 

 by which cohesion, friction, or inertia opposes the relative 

 motion of two elements. When the resisting element, or the 

 resistance due to the united action of two elements, gives rise to 

 a resisting couple, this resistance may be represented by two 

 ideal links between two elements. Thus, we see that in every 

 machine where there is only one source of power, we may re- 

 present that source by one ideal link (or two ideal links) con- 

 necting two elements, and acting on these elements as one 

 member (or two members) of a strained frame acts on the rest 

 of the structure. The name of driving link will be given to 

 ideal links of this kind. The name of resisting link will simi- 

 larly be given to ideal links used to represent the useful resist- 

 ance in a machine. In all cases the cause of motion and the 

 useful resistance must be considered as an action between two 

 elements. We cannot properly speak of a driving point or 

 resisting point, but only of driving or resisting links. 



7. Links. Dynamic Frame. Any actual material element 

 might at any instant be removed from a machine without 

 altering the stresses on the other elements, if at each joint thus 

 laid bare, force could be applied corresponding in position, mag- 

 nitude, and direction with the pressure supplied at that instant, 

 and at that joint by the element removed. The series of forces 

 supplied by any one element are necessarily such as would, when 

 balanced by equal and opposite forces, leave the element in equi- 

 librium. If, therefore, an element has only two bearing joints 

 and is without weight or inertia, the forces it supplies must 

 lie in one straight line, and either push or pull, as the member 

 of an actual frame does, when under simple tension or compres- 

 sion ; the element, in fact, acts like one link of a frame, as 

 shown in Fig. 2, where the element a might be replaced 1 by the 

 link 1, shown by the line on which arrow-heads are placed. 

 The ideal link replacing the actual element does not necessarily 

 or generally lie in the geometrical axis of the element. When 



1 The writer has iu this paper ventured to use the verb ' to replace ' as it is 

 usually employed by writers on chemistry, namely, as the translation of the 

 French word remplaccr. 



