f 3296 4 
the erroneous judgement which any 
body might form of the accurate 
degree of goodnefs of the air of any 
given place, by examining it once 
or twice with nitrous air, principally 
if the obfervator is not in poffeffion 
of anaccurateinftrument for making 
gach. an oan, or if he has not 
rved -nicety all 
the manoeuvres in ce ea or mak- 
ing the experiment. 
It would be a difficult tatk to 
difcover as yet the true caufes of 
that. continual fluctuation in the 
degree of falubrity of the air in 
the fame place. But it feems to- 
me not improbable, that this incon- 
ftancy is to be attributed in general 
to the natural. changeablenefs of 
the air itfelf, by which it under- 
goes continual alterations from 2 
variety of caufes, of which a great 
number 
