( 217 ) 
Fine Mercury (fuch as is made ufe of in Barome- 
ters) is, generally fpeaking, 137 times heavier than Wa- 
ter ; and, I found fome brought from the Eaft-Indies> 
to be 14 times heavier. I have found Air in Summer, 
to be near 900 times lighter than Water j and 800 
times in Winter \ therefore I take 850 at a Medium. 
Now 8 fo X 13} = 11606,6, which, divided by 120, 
gives 96,7 Feet, for fy of an Inch of Mercury , or 80,5 
Feet for i* of an Inch. This Number, taken invariable, 
will, in taking the Height of feveral Hills, agree pretty 
well with the Numbers that come out, when Dr. Halley's 
Table is made ufe of ^ and with the Experiment made 
by the late Profelfor, Mr. J. Cajwell , who, having 
taken the Height of Snowdon Hill in Caernarvan - 
J hire very accurately, and finding it to be 3720 Feet 
above the Level of the Sea, tried how much lower the 
Mercury wou’d (land in the Barometer upon that Hill, 
than at the Level of the Sea, and obferv’d it to fubfide 
3 9 Inches. I am fenfiblethat it will be alledged, that 
the Air will be denfer than I may imagine on the Top 
of high Hills, becaufe of the great Cold, fince they are 
generally cover’d with Snow ; but then we are to con- 
fider, that when we are got above a Mile higher than 
the Level of the Sea, the incumbent Atmofphere has loft 
almoft a 5th Part of its Weight * ; and therefore the Air 
at the Top of the Hill, being fo much lefs prefs’d, will, 
notwithftanding the intenfe Cold, be more rarifiedthan 
at the Bottom of the Hill. 
* If the whole Air was reduced to the fame Denfity, as it is near the Level 
of the Sea , the Atmofphere wou'd be but between five and fix Miles high ; where- 
as, in its prefent State of gradual Rare fab lion, it is above 50 Miles high, as we 
find by feveral Phenomena of Meteors obferv'd to float in the Air, fo hi^h at 
leaf. 
V o l. XXXIII. I i . Now 
