APPLICATION OF GRAPHIC METHODS 277 



frequently referred to, and will, for brevity, be spoken of as the 

 condition that the bearing pressure shall make the stated atigle 

 with the joint. 



6. Driving Element and Resist ing Element Driving Link and 

 Resisting Link. The source of power in a machine is often con- 

 tained in a single element, which is usually so combined with 

 the others as to exert equal and opposite forces in two directions, 

 and only in two directions. The steam in a cylinder of a steam- 

 engine affords one example of this kind of element, which will 

 be called a driving element. If the directions of the equal and 

 opposite forces exerted by the driving element lie in the same 

 straight line, the power tends to lengthen or shorten the element 

 in a definite direction. The two forces in this case tend to 

 separate or draw together the other elements with which the 

 first is jointed, as a straight elastic link would do if extended or 

 compressed so that its line of action coincided with the direction 

 of the forces exerted by the driving element. We may there- 

 fore conceive an actual driving element as replaced by an ideal 

 elastic link producing the same force at the above-named joints. 

 Some machines have no material driving element, but are ac- 

 tuated by an attraction or repulsion between two of their ele- 

 ments, as, for example, when a machine is driven by a weight. 

 In this case, as in the former case, we may conceive the machine 

 as driven by an ideal elastic link between two elements. This 

 link is as well suited to replace the equal and opposite forces 

 due to gravitation as the equal and opposite forces due to steam. 

 If the two opposite and equal forces exerted by a driving element 

 do not lie in one straight line, these forces exert a couple between 

 the elements with which the driving element is jointed; a 

 similar couple may be exerted by the forces of attraction or re- 

 pulsion between two elements. In either case, the machine 

 may be considered as actuated by an ideal couple due to the 

 elastic reactions of two links. What has been said of the driv- 

 ing power applies mutatis mutandis to the useful work done by 

 a machine. In some cases the work is actually done in lengthen- 

 ing or shortening a separate resisting element, as when a bale 

 of cotton is compressed. In other cases an attraction or re- 

 pulsion is overcome between two elements, as when a weight 

 is lifted. In both these cases we mav conceive the resistance 



