ASTRONOMY, ETC., OF SEVENTEENTH CENT. 209 



circles G D and B A rolling along the straight line G B. Let a flexible 

 but inextensible cord, c F, be attached to the point c, and let F be 

 taken in the cord so that E F equals c G ; then if the cord be kept tight 



T 



FIG. 92. 



FIG. 93. 



FIG. 94. 



FIG. 91. 



FIG. 95. 



while it is wound round the curve A c, the point F will describe another 

 cycloid, F H A, which is precisely the same as c A reversed. 



Huyghens, availing himself of this singular property, suspended the 

 rod of his pendulum by silken threads placed between cycloid blocks, 

 like c D, c A in Fig. 94, so that the threads should apply themselves 



14 



