Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 1.djvu/59

Book I. bodies, with which the comparion is made, that the relative poition may be preerved, then that condition will be preerv'd, in which the relative motion coniits. And therefore, any relative motion may be changed, when the true motion remains unalter'd, and the relative may be preerv'd, when the true uffers ome change. Upon which accounts, true motion does by no means conit in uch relations.

The Effects which ditinguih abolute from relative motion are, the forces of receding from the axe of circular motion. For there are no uch forces in circular motion purely relative, but in a true and abolute circular motion, they are greater or les, according to the quantity of the motion. If a veel, hung by a long cord, is o often turned about that the cord is trongly twited, then fill'd with water, and held at ret together with the water; after by the udden action of another force, it is whirl'd about the contrary way, and while the cord is untwiting it elf the veel continues for ome time in this motion; the urface of the water will at firt be plain, as before the veel began to move: but the veel, by gradually communicating its motion to the water, will make it begin enibly to revolve, and recede by little and little from the middle, and acend to the ides of the veel, forming it elf into a concave figure, (as I have experienced) and the wifter the motion becomes, the higher will the water rie, till at lat, performing its revolutions in the ame times with the veel, it becomes relatively at ret in it. This acent of the water hews its endeavour to recede from the axe of its motion; and the true and abolute circular motion of the water, which is here directly contrary to the relative, dicovers it elf and may be meaured by this endeavour. At firt, when the relative motion of the water in the veel was Rh