meaſures of them (either accurate or inaccurate) which are commonly uſed inſtead of the meaſur'd quantities them'elves. And if the meaning of words is to be determin'd by their uſe; then by the names Time, Space, Place and Motion, their meaſures are properly to be underſtood; and the expreſſion will be unuſual, and purely Mathematical, if the meaſured quantities themſelves are meant. Upon which account, they do ſtrain the Sacred Writings, who there interpret thoſe words for the meaſur'd quantities. Nor do thoſe leſs defile the purity of Mathematical and Philoſophical truths, who confound real quantities themſelves with their relations and vulgar meaſures.
It is indeed a matter of great difficulty to diſcover, and effectually to diſtinguiſh, the True motion of particular bodies from the Apparent: becauſe the parts of that immovable ſpace in which thoſe motions are perform'd, do by no means come under the obſervation of our ſenſes. Yet the thing is not altogether deſperate; for we have ſome arguments to guide us, partly from the apparent motions, which are the differences of the true motions; partly from the forces, which are the cauſes and effects of the true motions. For inſtance, if two globes kept at a given diſtance 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 tenſion of the cord, diſcover 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 ſhould be impreſs'd at once on the alternate faces of the globes to augment or diminiſh their circular motions; from the encreaſe or decreaſe of the tenſion of the cord, we might infer the increment or decrement of their
motions; and thence would be found, on what faces
