KINEMATIC MODELS. 279 



revolve with a uniform velocity, it gives to d the varying velocity 

 which has been so often utilised. The mechanism is obtained from 

 the same chain as before, simply by choice of a different link for the 

 stationary one* In general it is constructively disguised in such a way 

 as to be only recognisable with difficulty, but when put in this 

 schematic form it can easily be analysed into its kinematic elements. 



From the same chain, we may obtain one more mechanism by 

 fixing the block (Fig. 10). This mechanism has been seldom used, 



but it is occasionally employed. The motion of the link a is now 

 characteristic. 



We must now proceed to look at a few other leading ideas illustrated 

 by these models. First, we may notice as long as the form of the 

 elements of the pairs remains unchanged, their relative size is a 

 matter of indifference. We have already seen this incidentally in 

 comparing Figs. 3 and 5. By utilising this principle we can obtain a 

 great number of very different-looking forms of one and the same 

 mechanism. Models of many of these are on the table before me : 

 some of them are constantly used by engineers, others have been 

 seldom or never applied. The ordinary eccentric is a familiar illus- 

 tration of this " expansion of elements." It differs from the train 

 shown in Fig. 6 only in the relative size of two of its elements. The 

 turning-pair at 2 (Fig. 6) is made so large as to extend beyond the 

 pair i ; all the motions in the train remain, however, as before. 



