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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 pofTeflion 

 of an accurate inftrument for making 

 fuch an obfervation, or if he has not 

 obferved to the greaterr. nicety all 

 the manoeuvres in the time of mak- 

 ing the experiment. 



It would be a difficult talk 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 a 

 variety of caufes, of which a great 



number 



