t 2 44 ] 
SECTION FOURTH. 
Of the equati©n for the temperature of the air. 
I F the temperature of the atmofphere hath been 
any other than is expreffed by -f- i6| of M. de 
luc’s fcale, the refult of the calculation, formed 
upon the preceeding rules, requires a further cor- 
redion. This corredion arifes from a variation of 
the length of the fubtangent of the atmojpherical loga- 
rithmic j which, as hath been already remarked, is 
found to be net conftantly the fame. That this mat- 
ter may be the better underftood, it will be proper 
to date, in this place, what this fubtangent is, in the 
nature of things, and upon what phyfical circum- 
ftances its length depends. 
Imagine then, thatinfteadof the atmofphere, in its 
natural date, the earth were furrounded with an ine- 
laftic fluid, of an uniform denfity throughout, equal 
to that of the natural atmofphere, in its lowed: parts, 
which are contiguous to the furface of the earth ; ima- 
gine alfo, that every atom of this homogenous fluid 
were urged towards the earth’s center, with an accelera- 
tive force, equal to that of gravity, at the furface of the 
earth. Now the preffure of the atmofphere upon the 
earth’s furface, or on any given part thereof, being, at 
all times, a finite, though not a conftant force, of which 
the phenomena of the Torricellian tube are a fuf- 
fleient proof, it is evident that it might, at any time, 
be equalled by the preffure of a finite quantity of 
this imaginary fluid ; and that, to render the preffure 
of this fiditious atmofphere, upon the whole, or any 
part of the earth’s furface, equal to that of the real 
atmofphere upon the like part, it would be requiflte 
to afiign to it fome finite thicknefs or depth. Now 
