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mating the variations which the denfity of the air, 
at any place, undergoes ; and the other for deter- 
mining the fpecific gravity of the air of any tem- 
perature, at any elevation. The fixth and laft 
fedtion points out certain confequences, which feem 
to follow from the difcoveries already made in con- 
junction with the theory eftablifhed. Thefe are 
only propounded to awaken curiofity, and promote 
enquiry. I am well aware, how little theory is to 
be trufted, in its remote conclufions, on account of 
the neceflary deficiencies of the phylical data , upon 
which its reafonings are founded. The true ufes of 
it are, either to explain the mutual connexions and 
dependencies of things already known, or to fuggeft 
eonjedtures concerning what is unknown, to be tried 
by future experiment. And he who applies it, with 
due circumfpedfion, to thefe purpofes, will always 
find it an ufeful engine. I flatter myfelf, that I have 
affigned the true caufe of fome very Angular pheno- 
mena, remarked by M. del uc. I have endeavoured, 
to treat every part of my fubjedf, in theplainefl: man- 
ner, that the nature of it would admit ; and fo dif- 
fufely, as to be, I hope, intelligible to all, who are 
moderately well founded in the mathematical fciences; 
for I have obferved with pkafure, that M. de Luc’s 
book hath raifed a general curiofity upon the points 
it treats of j and for that reafon, it feemed the more 
neceflary, to depart in this inftance, from a practice 
of late become too general, with mathematicians, to 
write only for one another. To which, if I am not 
miftaken, it is in great meafure owing, that thefe 
noble and ufeful ftudies are lefs generally attended 
to, than they were in former times; when men of 
eminence, 
