Bodies being compressable into a very small Space, have yet a determinate bulk which they cannot exceed, when free'd from all manner of Pressure.
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