276 SECTION MECHANICS. 



by the use of the slot and sector of Fig. 5 instead of the pin and eye 



Figure 5. 



g h (Fig 3), no change has been made in the chain. The complete 

 cylinder pair has been replaced by a sector and a slot concentric with 

 it, but of totally different diameter ; the motions remain absolutely 

 identical. We obtain thus a very convenient method of altering 

 the length of the links of the chain, for the link f g has now 

 taken the form of the sector c (Fig. 5). To make that link longer, 

 herefore, all we have to do is to give the sector a larger radius. If we 

 make the link infinitely long, the sector becomes a prism, working in a 

 straight slot of which the axis passes through the centre of the pin i 

 (Fig 5), and we obtain this very familiar chain (Fig 6), the driving 



Figure 6. 



mechanism of the common steam engine. It is derived from the 



