in order to afcertain the height of Mountains . 565 
uniform temperature, which in my room I found not 
very eafy to effect in heats greater than 70° or 8o°. I 
have therefore preferred repeating the experiment with 
fmall differences of heat; but fuch, however, as will in- 
clude almoft all the temperatures in which barometrical 
obfervations are likely to be made, viz. from 32 0 to 83°, 
Xt has been fufpefled, in confequence of fome experi- 
ments made by a very ingenious member of this Society, 
that air does not expand uniformly with quickfilver ; or 
that the degrees of heat fhewn by a quicklilver-ther- 
mometer would be expreifed on a manometer, or air- 
thermometer, by unequal fpaces in a certain geometrical 
ratio. I do not deny this propofition ; but I have alio 
very little reafon to aflent to it, if I may trull my own 
experiments, which certainly evince that this ratio, if 
not truly arithmetical, is fo nearly fo as to occalion no, 
fgnfible error in the meafuring of heights with the ba- 
rometer; and that is all I contend for. The fmall dif- 
ferences that are feen in the above table of this expan- 
fion, deduced from a mean of 14° or of 40°, I would 
attribute rather to the errors of obfervation than to any 
a,6lual irregularity in nature. If, however, this progrei- 
fion be infilled upon, it fhould feem, that the. degree of 
the air’s expanlion increafes with an increafe of heat; 
and that the difference of volume or denfity ' torn 1 : ot 
