ON PENDULUMS. ff £ 



calls the circular pendulum, and which much refembles the Circular pendu- 

 centriFugal apparatus, ufed frequently about fleam-engines to hmofHuygens. 

 regulate the aperture of the fteam-darnper, of which it pro- 

 bably fuggefted the firft idea. To form the circular pendu- 

 lum, a fpindle (H D, Fig. 1, Plate III.) proceeds perpen- 

 dicularly from the clock-work, from whence it receives a cir- 

 cular motion round its own axis; to this is affixed a lamina 

 (BGA) of fome breadth, bent according to a paraboloidal 

 Ji/ie, by the evolution of which (after it is joined to a certain 

 right line) a parabola is formed, the conftniclion of which is 

 (hewn in the eighth proportion of the third part of the above 

 work : This lamina caufes the ball of the pendulum (F), at- 

 tached by two threads to its upper extremity (as it circulates), 

 to perform all its revolutions (which will be of greater or lefs 

 extent as the axis moves with greater or lefs force) in the fur- 

 face of a conoidal parabola (F E). Huygens declares, that 

 all the circles performed by the pendulum thus conftructed, 

 will be ifynchronous, and then mews how to proportion the 

 parts of the apparatus, fo as that each of its revolutions Qia.Il 

 be performed in a fecond, or in half a fecond : He fays, the 

 only reafon which caufed the ofcillatory pendulum to be pre- 

 ferred to this was, that this la ft was more difficult to conftrucV. 

 As this circular pendulum feems to poffefs the valuable pro- 

 perty of correcting the alterations caufed in it by expanfion, 

 or of rendering them of no confequence, the difficulty of con- 

 struction is a matter of no confequence, efpecially as there are 

 now to be found arlifts fo much more excellent in works of 

 this kind than formerly. 



Huygens gives the following character of this pendulum, 



1 which I tranferibe in his own words : " Plura tamen, hujus 

 qunpie generis de quo nunc loquimur, nee fine Jucceffu, conjlrutia 



■. fuere : eft que in his fingulare Mud, quod continuo atque ctqua- 

 bili motu circumferri cernitur index pnjiremus, qui ftcunda fcru- 

 pula dejignat, cum in omnibus aliis horologiis fubfultim quafi 

 feratur ; Item hoc quoque, quod abfque ftrepitu jbnoque omni 

 moveantur hoc ratione conjlructa automata." He concludes the 

 book with thirteen theorems, De vi centrifuga ex motu circu- 

 lari, feveral of which prove this kind of pendulum to have 

 moft valuable qualities : the fixth of thefe, being; very remark- 

 able, I copy for your readers, who may not have an oppor- 

 tunity of feeing the work which contains it, as follows : 



" THEOREMA. 



