Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 1.djvu/58

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A property near a kin to the preceding, is this, that if a place is mov'd, whatever is placed therein moves along with it; and therefore a body, which is mov'd from a place in motion, partakes alo of the motion of its place. Upon which account all motions from places in motion, are no other than parts of entire and abolute motions; and every entire motion is compoed out of the motion of the body out of its firt place, and the motion of this place out of its place, and o on; until we come to ome immovable place, as in the before mention'd example of the Sailor. Wherefore entire and abolute motions can be no otherwie determin'd than by immovable places; and for that reaon I did before refer thoe abolute motions to immovable places, but relative ones to moveable places. Now no other places are immovable, but thoe that, from infinity to infinity, do all retain the ame given poitions one to another; and upon this account, mut ever remain unmov'd; and do thereby contitute, what I call, immovable pace.

The Caues by which true and relative motions are ditinguihed, one from the other, are the forces impres'd upon bodies to generate motion. True motion is neither generated nor alter'd, but by ome force impres'd upon the body moved: but relative motion may be generated or alter'd without any force impres'd upon the body. For it is ufficient only to impres ome force on other bodies with which the former is compar'd, that by their giving way, that relation may be chang'd, in which the relative ret or motion of this other body did conit. Again, True motion uffers always ome change from any force impres'd upon the moving body; but Relative motion does not necearily undergo any change, by uch forces. For if the ame forces are likewie impres'd on thoe other Rh