THE KINEMATICS OF MACHINERY. 77 



technologically and teleologically. Then there comes what 

 we may call the study of prime-movers, which in terms of 

 our definition would "be the study of the arrangements by 

 means of which the natural forces can be best compelled to do 

 the required work. Then comes the study of what may be 

 called " direct-actors," or the direct-acting parts of machinery ; 

 in the terms of our definition, the arrangement of the parts 

 of a machine in such a way as best to obtain the required result. 

 Next comes what we call machine design ; the giving to the 

 bodies forming the machine the requisite quality of resist- 

 ance. Machine design is based principally on a study of 

 the strength of materials. 



One clause of the definition still remains untouched. The 

 machine, we said, does work accompanied by certain deter- 

 minate motions. Corresponding to this we have in machine 

 instruction the study of those arrangements in the machine by 

 which the mutual motions of its parts, considered as changes of 

 position only, are determined. The limitation here must be 

 remembered ; motion is considered only as change of posi- 

 tion, not taking into account either force or velocity. This 

 is what Professor Willis long ago called the " science of pure 

 mechani-m," what Eankine has called the " geometry of 

 machinery," what Reuleaux calls '''kinematics," and what 

 I mean now by the " kinematics of machinery." 



The resets of many years' work of Eeuleaux in connection 

 with this subject are embodied in his book Die Theoretische 

 Kinematik, which I recently had the pleasure of trans- 

 lating, and I shall endeavour to give you an outline* of his 

 treatment of the subject. It cannot be more than an outline, 

 as you will readily understand. The subject is a very large 

 one, and I have had to choose between taking up many branches 

 of it and merely mentioning each, and confining myself to a few 

 points, and going more into detail about them. I have chosen 

 the latter plan, believing that the former would be of little 

 benefit to anybody. It will be easy for those who are 

 sufficiently interested in the matter tc follow it up, and to 

 study those parts which I omit by the aid of the book I 

 have just mentioned. My lecture to-day will be principally 

 theoretical, and to-morrow I shall go more into practical 

 applications. So far as possible, as I have Professor 

 Keuleaux's models before me, I shall endeavour to follow his 

 own order in treating the subject. 



