96 LECTURES TO SCIENCE TEACHERS. 



We have already noticed that the chain has four links. We 

 see further that it is a chain in which all the motions are con- 



Fio. 10. 



plane, each of its four pairs being simply a cylinder pair, and 

 the four cylinder pairs having parallel axes. It is so propor- 

 tioned that by causing one link to swing, another one can be 

 made to revolve. In order that we may refer more easily to 

 the links a letter is attached to each in the engraving. 



For convenience sake we may also use a short symbol for 

 this chain (the one used by Reuleaux) namely, ((/i) 1 The (7 4 

 within brackets stands for the four cylinder pairs, the symbol 

 for parallel being added to indicate their relative positions. 

 This is the symbol for the chain, no link being fixed. To 

 distinguish the four mechanisms formed from it, we shall put 

 the letter which stands for the fixed link in the position of 

 an index after the formula. Thus we can denote the parti- 

 cular mechanism shown in Fig. 10, in which the link d is 

 fixed, by the formula (Cl) d . We have here then the first of 

 the four mechanisms we can get from this chain. You will 

 recognize it easily enough as exactly similar to the beam and 

 crank of a beam-engine. The link c is half the beam, a the 

 crank and b the connecting-rod. The whole mechanism is an 

 excellent illustration of what I said in my last lecture, that the 

 form of the links is indifferent. If you think of the mecha- 

 nism as forming part of a beam-engine, for instance, you will 

 see in the link d the abstract form of what is generally a most 

 complex structure, a bed-plate with its bearings, an entabla- 

 ture and plummer-block, cast-iron columns, and in some cases 

 even brick and masonry. All these are represented by the 

 fixed link d so far as their kinematic relations are concerned. 



If now we fix the connecting-rod b instead of fixing the 



1 In words " C parallel 4," 



