I 2&7 1 
tent ; as this is the only one method \tfhich gives 
entire fetisfadtion to the- mind. 
SECTION SIXTH. 
I HAVE now gone through the expofition and 
reduction of M. de luc’s rules, for meafuring 
heights by the barometer. The confonance of them 
with theory appears, upon a ftridt examination, to 
-be fuch, as ftrongly confirms the principles, upon 
which the theory is founded. 1 (hall conclude thefe 
fdifquifitions, with pointing out fome further objedts 
of enquiry., concerning the modifications of the at- 
:tnofphere, naturally arifing out of M. de iuc’s 
difcoveries, in conjunction with the theory already 
cftablifhed. 
i. It is probable , that the abfolute elaJHcity of 
the air may be ajfedled by various caufes befdes 
heat . The degree of humidity muft occur to every 
one, as a circumftance, which may reafbnably be 
Cufpedfced to have feme influence upon it ; and, per- 
haps, the ftate and quantity of -the eledtricity of the 
air may have more. 
2 . If M. de luc’s formulae are to be admitted 
as univerfally true, in all imaginable temperatures, 
• there is a given temperature in which the elafticity 
of the air would be dejlroyed, and , in any lower tem- 
perature , it would be negative ; that is, the re - 
pulfion would be changed into attraction. This 
given temperature is, — 409*13 of bird’s Fah- 
renheit; for if B 4“ B, be the length of the 
Mm 2 fubtangent 
