INTRODUCTORY

DEFINITIONS AND REMARKS.

Fig . i. An arch of equipollencc is a curve,whofe feveral parts are prevented from follow-ing their natural directions towards the centreof the earth by mutual oppofition. Therefpe&ive a&ions of the parts are fo regulated,that through their reciprocity they mull re-main in their aClual If ate; or, if they begin togravitate at the fame time, each part muftmove through the fame perpendicular fpace inthe fame time, and the chord A B will alwayskeep its parallelifm. It is, therefore, of no im-portance to the liability or form of the arch ofwhat confiftency the foundations A, B are; lothat they are equally compreffible, and defcendtogether, keeping their horizontal plane. Un-lefs the parts A, B move, no other part canmore.

b

If