336 
Thus if A instead of sliding on a guide be attached by a 
pin to the extremity of a long link AK, whereof the other 
extremity AK is moveable round a fixed pin k, A will describe 
a small portion of the arc of a large circle, and therefore move 
approximately in a straight line. This is in fact a case of 
Watt's parallel motion. The fixed pomt & might have been 
above the line 4D. 
_ It is to be observed that C will describe an approximate 
straight line for a considerable space, if A for a short space 
describes an approximate straight line. 
Hence we have only to make A move in a straight line for 
a short space by any means we can: for instance, by means of 
Watt’s parallel motion. 
The above remarks will explain the rationale of the follow- 
ing model in linkwork in the late Prof. Willis’ collection. 
ACC'A is a parallelogram hinged at A, C, C’, A’; B bisects 
