_Fifthly_, That this Power increases as you descend, and decreases as you ascend from the Center, and that in the Proportion of the Squares of the _Distances_ therefrom _reciprocally_, so as at a double Distance to have but a quarter of the Force; this Property is the Principle on which Mr. _Newton_ has made out all the _Phænomena_ of the _Cœlestial Motions_, so easily and naturally, that its Truth is past Dispute. Besides that, it is highly rational, that the _attractive_ or _gravitating_ Power should exert it self more vigorously in a small Sphere, and weaker in a greater, in proportion as it is contracted or expanded; and if so, seeing that the _Surfaces_ of _Spheres_ are as the _Squares_ of their _Radii_, this Power, at several Distances, will be as the _Squares_ of those _Distances reciprocally_; and then its whole Action upon each _Spherical Surface_, be it great or small, will be always equal. And this is evidently the Rule of _Gravitation_ towards the _Centers_ of the _Sun_, _Jupiter_, _Saturn_ and the _Earth_, and thence is reasonably inferred, to be the general Principle observed by _Nature_, in all the rest of the _Cœlestial Bodies_. These are the principal Affections of _Gravity_, from which the Rules of the _Fall_ of _Bodies_, and the _Motion_ of _Projects_ are _Mathematically_ deducible. Mr. _Isaac Newton_ has shew'd how to define the Spaces of the _Descent_ of a _Body_, let fall from any given height, down to the _Center_, supposing the _Gravitation_ to increase, as in the fifth Property; but considering the smallness of heighth, to which any _Project_ can be made ascend, and over how little an _Arch_ of the _Globe_ it can be cast by any of our _Engines_, we may well enough suppose the _Gravity_ equal throughout, and the Descents of _Projects_ in parallel Lines, which in Truth are towards the _Center_, the difference being so small as by no means to be discovered in _Practice_. The _Opposition_ of the _Air_, 'tis true, is considerable against all light Bodies moving through it, as likewise against small ones (of which more hereafter) but in great and ponderous Shot, this Impediment is found by _Experience_ but very small, and may safely be neglected. _Propositions concerning the Descent of Heavy Bodies, and the Motion of _Projects_._ _Prop. I._ The _Velocities_ of _Falling Bodies_, are proportionate to the Times from the beginning of their _Falls_.