[ 2I& ] 
with my firfic defign, takes up the far greater part of 
them. They can hardly be free from the imper- 
fections naturally incident to productions, in which 
the plan hath been gradually changed, during the 
actual progrels of the work. I flatter my self, how- 
ever, that they are not deficient in two dfential 
points, the precifion and the perfpicuity of the ma- 
thematical reafoning; and that, however unfinifhed 
in fome particulars, they are fuch upon the whole, 
as the dignity of the fubjeCt may, in fome degree, 
commend to your protection. There is perhaps no 
branch of phyfical enquiry, intrinfically more fubiime 
and interesting, nor likely to be more important in 
its ufes, than that which immediately regards the 
conftitution of that elaftic fluid, which lurrounds 
our globe, and appears to be a principal agent, in 
many of the molt flriking operations of nature, 
and a neeeffary inffcrument, at leaft, in carrying on 
the wonderful bufinefs of vegetation and of animal 
life. Upon a fubjedt of fo much importance, it mud: 
afford fatisfaclion, to find fo exact an agreement, as 
is evinced, if I am not much deceived, in the enfuing 
pages, between a multiplicity of experiments, not 
luggefted by any previous Speculations of theory, 
with a theory, whole conclufions in this branch had 
never before been duly fubmitted to the teft of ex- 
periment. 
The whole of the following paper is divided into 
fix feCtions. The bufinefs of the firft, is merely 
preparation for the principal work. It contains a 
brief account of the fum of Mr. de luc’s re- 
fearches concerning the variation of the heat of 
2 boiling 
