C 25° ] 
tangent of the curve, defcribed in fe&ion fecond, will 
vary : for the difference of the lubtangents is the 
only thing that conftitutes a real difference in two 
logarithmic carves. If iuch carves have equal lub- 
tangents, though they have unequal ordinates, they 
are only different paints of the fame curve. 
The fubtangent of the aimofphericai logiffic muff 
always be as the preffure of the whoie column of 
the atmofphere direftly, and the denfity, at the 
earth’s furface, inverfely ; therefore it is diredly as 
the abfolute eiaftic force. For call the fubtangent S ; 
the preffure at earth’s furface, P j the deniity, D ; 
the abfolute elafticity, A. 
P 
Now that S is as ^ is obvious (from p. 245.); 
p 
but ^ is as A (by what hath now been proved). 
Therefore S is as A. 
Now heat W is one caufe, which is well known to 
influence the abfolute elaftic force. An increafe of 
heat increafes elaflicitv j and elafticity is diminifhed 
by a diminution of heat. Accordingly M. de luc’s 
experiments fhew, that when the temperature of 
the air is uniform (or the fame at all heights) 
the fubtangent of the atmofpherical curve is in- 
creafed, or diminifhed, exactly in proportion to the 
increment or decrement of the uniform tempera- 
ture, as indicated by the mercurial thermome- 
ter. His conclufion, from repeated experiments 
en the mountains near Geneva, is, that, if L de- 
note the difference of the tabular logarithms off the 
heights of the quickjiher , at two fat ions, corrected 
for the difference of the temperature of the quick- 
(/>) boerhaaye. Elements Chemise, vol. i, p.456, &c. 
fiver } 
