Now in order to the giving account of these three Periods, according to the _Laws of Motion_ and _Mechanick Principles_; We shall _first_ take for granted, what is now adayes pretty commonly entertained by those, who treat of such matters; _That a Body in motion is apt to continue its motion, and that in the same degree of celerity, unless hindred by some contrary Impediment_; (like as a Body at rest, to continue so, unless by some sufficient mover, put into motion:) And accordingly (which daily experience testifies) if on a Board or Table, some loose incumbent weight, be for some time moved, & have thereby contracted an _Impetus_ to motion at such a rate; if that Board or Table chance by some external obstacle, or otherwise, to be stopped or considerably retarded in its motion, the incumbent loose Body will shoot forward upon it: And contrarywise, in case that Board or Table chance to be accelerated or put forward with a considerably greater speed than before, the loose incumbent Body, (not having yet obtained an equal _Impetus_ with it) will be left behind, or seem to fly backward upon it. Or, (which is _Galilaeo_'s instance,) if a broad Vessel of Water, for some time evenly carried forward with the water in it, chance to meet with a stop, or to slack its motion, the Water will dash forward and rise higher at the fore part of the Vessel: And, contrarywise, if the Vessel be suddenly put forward faster than before; the Water will dash backwards, and rise at the hinder part of the Vessel. So that an Acceleration or Retardation of the Vessel, which carries it, will cause a rising of the Water in one part, and a falling in another: (which yet, by its own weight, will again be reduced to a Level as it was before.) And consequently, supposing the Sea to be but as a loose Body, carried about with the Earth, but not so united with it, as necessarily to receive the same degree of _Impetus_ with it, as its fixed parts do: The acceleration or retardation in the motion of this or that part of the Earth, will cause (more or less, according to the proportion of it) such a dashing of the Water, or rising at one part, with a Falling at another, as is that, which we call the Flux and Reflux of the Sea. {269} [Illustration] Now this premised, We are next, with him, to suppose the Earth carried about with a double motion; The one _Annual_, as (_Fig._ 1.) in B E C the great Orb, in which the Center of the Earth B, is supposed to move about the Sun A. The other _Diurnal_, whereby the whole moves upon its own _Axis_, and each point in its surface describes a Circle, as D E F G.