104 



LECTURES TO SCIENCE TEACHERS. 



We must pass over without mention many other modifi- 

 cations and alterations of the chain, and mention only one 

 other form in which it occurs, a form which has some special 

 interest. The condition of movability of a chain that 

 contains four cylinder pairs is not that their axes should be 

 parallel, but that they should meet in one point. The axes 

 are parallel only in the special case where this point is at an 

 infinite distance. Fig. 19 is an illustration of the more 

 general, although less familiar, case when the point of inter- 

 section is at a finite distance. This chain, which may be 



Fio. 18. 



indicated by the formula (CJ-) l has again its four inversions, 

 and furnishes us with three different mechanisms as in the 

 case of (C'). I cannot here go into these ; I mention the 

 chain partly because of its theoretic interest, and partly 

 because although it looks so unfamiliar, it is not unfrequently 

 applied in machinery. If instead of subtending only a small 

 angle, as in Fig. 1 9, three of the links were made to subtend 

 an angle of 90, we should have the common Hooke's or 

 universal joint. In these "conic" chains, links which are 

 quadrants take the place of the infinite links, so that a chain 

 having three quadrants corresponds to the chain of Fig. 7, 



1 "C four oblique." 



