( }-o8 ) 
III. ObferVations concerning the Height of the Baro- 
meter, at different Elevations above the Surface of 
the Earth , in a Letter to the Tublifher from the 
Learned Dr. Netdeton. 
SIR , 
B EING curious to learn by Obfervation, how far 
the Mercury will defcend in the Tube at any given 
Elevation, for which there is fufficient Opportunity 
hereabouts, I propofed to take the Altitude of fome 
of our higheft Hills j but, when we attempted it, we 
found our Obfervations fo difturbed by Refra&ions, that 
we cou’d come to no Certainty. Having meafur’d one 
Hill of a confiderable Height, in a clear Day, and ob- 
ferved the Mercury at the Bottom and at the Top, we 
found, according to that Eftimation, that about 90 Feet, 
or upward?, were required to make the Mercury fall one 
Tenth of an Inch ; but coming afterwards to repeat the 
Experiment on a cloudy Day, when the Air was fome- 
what grofs and hazy, we fouftd the fmall Angies fo 
much augmented by Refra&ion, as to make the Hill 
much higher than before, tho’ they were taken carefully 
with very good Inftruments, both at that Time and be- 
fore. 1 afterwards frequently obferv’d at home, by 
pointing the Quadrant to the Tops of fome of our neigh- 
bouring Mountains, that they wou’d appear higher in 
the Morning before Sun-rife, and alfo late in the Even- 
ing, than at Noon, in a clear Day, by feveral Minutes: 
Particularly, one Morning in 'December laft, when the 
Vapours lay condens’d in theVallies, and the Air above 
was very pure, the Top of a Mountain, at fome Diftance 
from 
