88 Mifcellanea Curiofa. 



of the Air from any aflign'd height of the Ba- 

 rometer at the Earth's Surface, and any fpecifick 

 Gravity given. As to the Condensation and 

 Rarif action by Heat and Cold, and the various 

 mixture of Aqueous and other Vapours, thefe 

 two Objections feem generally to compenfate 

 each other ; for when the Air is rarified by Heat, 

 they are raifed mod copioufly ; fo that though 

 the Air properly fb cal I'd, be expanded, and con- 

 (equenrly lighter, yet the Interftices thereof being 

 crouded full of Vapours of much heavier Mat- 

 ters, bulk for bulk, the weight of the Compofi- 

 tum may continue much the fame ; at leaft a 

 mofl curious Experiment made by the Ingenious 

 Mr. John CafwcII, of Oxford* upon the top of 

 Snovpdon Hill, in Carmrvanfhire, feems to prove, 

 that the firft Inches of Mercury have their Por- 

 tions of Air near enough to what I now deter- 

 mine : For the height of the Hill being 1 240 

 Yards, or very near it, he found the Mercury 

 to have fubfided to 2^,6 Inches, or 4 Inches be- 

 low 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 anfwering to 4 Inches, by my 

 Calculation, Ihould be 1288 Yards ; and it a- 

 grees as weU with the Obfervations in the Ap- 

 pendix to Mv. PafcaUh Book, del Equilibre des 

 Liqueurs, made on the high Hill in Auvergne 9 

 calld le puy de Dommc. So that the Rarifa&i- 

 on and Vapours feem not to have altered con- 

 fiderably, the Gravity of the under Parts of the 

 Air ; and much above the height where thefe 

 Experiments were made, do few Vapours a£ 

 cend, and the Cold is fuch that the Snow lies 

 continually, fo that for the more elevated Parts of 

 the Sphere of Air 9 there is much lefs Rea(6n to 

 doubt. But 



