These Objections being true, do disturb the Geometrical Accuracy of these Conclusions, drawn from the specifick Gravity of the Air observ'd at any time; but the Method here shewn will compute by a like Calculation, the heights of the Quick-silver, and the Rarifactions of the Air from any assign'd height of the _Barometer_ at the Earth's Surface, and any specifick Gravity given. As to the Condensation and Rarifaction by Heat and Cold, and the various mixture of Aqueous and other Vapours, these two Objections seem generally to compensate each other; for when the Air is rarified by Heat, they are raised most copiously; so that though the Air properly so call'd, be expanded, and consequently lighter, yet the _Interstices_ thereof being crouded full of Vapours of much heavier Matters, bulk for bulk, the weight of the _Compositum_ may continue much the same, at least a most curious Experiment made by the Ingenious Mr. _John Caswell_, of _Oxford_, upon the top of _Snowdon_ Hill, in _Carnarvanshire_, seems to prove, that the first Inches of _Mercury_ have their Portions of Air near enough to what I now determine: For the height of the Hill being 1240 Yards, or very near it, he found the _Mercury_ to have subsided to 25,6 Inches, or 4 inches below the mean Altitude thereof at the Level of the Sea, (which is a greater difference than has been found in any of our former Experiments,) and the Space answering to 4 Inches, by my Calculation, should be 1288 Yards; and it agrees as well with the Observations in the Appendix to Mr. _Pascall_'s Book, _del Equilibre des Liqueurs_, made on the high Hill in _Auvergne_, call'd _le puy de Domme_. So that the Rarifaction and Vapours seem not to have alter'd considerably, the Gravity of the under Parts of the _Air_; and much above the height where these Experiments were made, do few Vapours ascend, and the Cold is such that the Snow lies continually, so that for the more elevated Parts of the Sphere of _Air_, there is much less Reason to doubt.