202 Mechanic Powers. Book IX.

And this irregularity of Vibrations may proceed from athreefold head; the firſt is, the various temper of the air thro'Which the globe paſſes, for if it be thick it reſiſts more, if hea-vier, the motion is not only conſequently hipdred by the grea-ter denſity, but alſo the weight of the globe is rendred leſsPonderous, that therefore they have leſs ſtrength to cut theair, and it renders the motion of the wheels and the Impetusmore faint and weak. The ſecond is, the impulſive force ofthe Wheeh, or other power, continuing the vibrations, whichif ic be irregular, induces in like manner at leaſt ſome, if notall, the Jrregularity in the vibrations, and theſe two heads maybe referred to out ward cauſes. The third is from within, orfrom the nature of the vibrations themſelves, Which are, asWe have faid, that if a globe fall by its own weight, in ſwing-ing it will make many vibrations, yet they will not all be of alike continuance. 2 5

The irregularity of Vibrations which proceed from the firſthead may wholly be taken away, if the Clock with the Pen­ dulum be included in à caſe of glaſs, or ſuch like receptacle,wholly unpaſſable to the air(wich how eto perform we ſhallſhew hereafter.) wherefore it remains ihat we ſpeak to tlietwo other.

To the Irregularity af the third head, Which is as it wereinbred in the ſimple vibrations themſelves, 2 remedy ſeems tobe found by the induſtrious Hagenius by the following inge-nious practice, the whole Art conſiſts in a double arch of thedigure of a Cycloid, within which the Pendulum agitates.

Fig. 141. Let the pendulum be A B, which js agitated toand fro by indifferent arches B C, BD of 20 degrees, and letthis Pendulum hang by a thred or ſniall cord A F of one inchmore or leſs, accoi ding to the length of the Pendulum , whoſeremaining part E B müſt be ſtiff; diſpoſe here and there ncarthe centre A Ww O bowed thin plates f mettal AG H and AFI,40 that they touch the lines A D, A C, in the points F and G,and the cavity onght to have the figure ofa Cychoid; ſo it willcome to paſs that if the Pendujum make greater vibrations, andruns out bey ound the lines A C. A D, it will touch the Cycloidin the points FE and G, and is length will not any longer be A Dbut f D, and ſo when the Pendulum is made ſhorter, its vi-bralion comigues Ieſs time: But why the Author choſe rather

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die figure Of a Cycidid chan another Curve doth not ſufficient-