Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 1.djvu/61

Book I. meaures of them (either accurate or inaccurate) which are commonly ued intead of the meaur'd quantities them'elves. And if the meaning of words is to be determin'd by their ue; then by the names Time, Space, Place and Motion, their meaures are properly to be undertood; and the expreion will be unuual, and purely Mathematical, if the meaured quantities themelves are meant. Upon which account, they do train the Sacred Writings, who there interpret thoe words for the meaur'd quantities. Nor do thoe les defile the purity of Mathematical and Philoophical truths, who confound real quantities themelves with their relations and vulgar meaures.

It is indeed a matter of great difficulty to dicover, and effectually to ditinguih, the True motion of particular bodies from the Apparent: becaue the parts of that immovable pace in which thoe motions are perform'd, do by no means come under the obervation of our enes. Yet the thing is not altogether deperate; for we have ome arguments to guide us, partly from the apparent motions, which are the differences of the true motions; partly from the forces, which are the caues and effects of the true motions. For intance, if two globes kept at a given ditance one from the other, by means of a cord that connect them, were revolv'd about their common centre of gravity; we might, from the tenion of the cord, dicover the endeavour of the globes to recede from the axe of their motion, and from thence we might compute the quantity of their circular motions. And then if any equal forces hould be impres'd at once on the alternate faces of the globes to augment or diminih their circular motions; from the encreae or decreae of the tenion of the cord, we might infer the increment or decrement of their motions; and thence would be found, on what faces Rh