Now these Rules were rigidly true, were it not, as I said before, for the Opposition of the Medium, whereby not only the direct imprest Motion is continually retarded, but likewise the increase of the _Velocity_ of the _Fall_, so that the Spaces described thereby, are not exactly as the Squares of the Times: But what this Opposition of the _Air_ is, against several _Velocities_, _Bulks_, and _Weights_, is not so easie to determine. 'Tis certain that the weight of _Air_ to that of _Water_, is nearly as 1 to 800, whence the weight thereof, to that of any _Project_ is given; 'tis very likely, that to the same _Velocity_ and _Magnitude_, but of different Matter, the _Opposition_ should be _reciprocally_ as the weights of the Shot; as likewise that to Shot of the same _Velocity_ and Matter, but of different _Sizes_, it should be as the _Diameters reciprocally_: Whence generally the _Opposition_ to Shot with the same _Velocity_, but of differing _Diameters_, and _Materials_, should be as their _Specifick Gravities_ into their _Diameters reciprocally_; but whether the _Opposition_, to differing _Velocities_ of the same Shot, be as the _Squares_ of those _Velocities_, or as the _Velocities_ themselves, or otherwise, is yet a harder Question. However it be, 'tis certain, that in large Shot of _Metal_, whose weight many Thousand times surpasses that of the _Air_, and whose force is very great, in proportion to the _Surface_ wherewith they press thereon; this _Opposition_ is scarce discernable; For by several _Experiments_ made with all Care and Circumspection with a _Mortarpiece_, Extraordinary well fix'd to the Earth on purpose, which carried a solid Brass Shot of four Inches and a half _Diameter_, and of about fourteen Pound Weight, the _Ranges_ above and below forty five _Degrees_ were found nearly equal; if there were any difference, the under _Ranges_ went rather the farthest, but those differences were usually less than the Errors committed in ordinary Practice, by the unequal Goodness and Dryness of the same sort of Powder, by the Unfitness of the Shot to the Bore, and by the Loosness of the Carriage. In a smaller Brass-Shot of about an Inch and half Diameter, cast by a Cross-Bow which ranged it, at most about four Hundred Foot, the Force being much more equal than in the Mortarpiece, this difference was found more Curiously: and Constantly and most Evidently, the under Ranges out-went the upper. From which Trials I conclude, that although in small and light Shot, the Opposition of the Air, ought and must be accounted for; yet in Shooting of great and weighty Bombs, there need be very little or no allowance made; and so these Rules may be put in practice to all Intents and Purposes, as if this Impediment were absolutely remov'd. _A Proposition of general Use in the Art of Gunnery, shewing the Rule of laying a Mortar to pass, in order to strike an Object above or below the Horizon._