ON LAND AND SEA AIR. 265 



prepare to meet with oppofition, particularly from thofe 

 who have formed opinions from reafoning alone, unfup- 

 ported by experiments. In doubtful matters it is chiefly 

 by the clafliing of opinion, that truth is hnally diicover- 

 ed. This Ihall be both my confolation and apology, if 

 the refult of my experiments (hall be found to have in- 

 duced me to diifer from others. Neverthelefs it will af- 

 ford me confiderable fatisfafticn to agree with thofe 

 whofe decifions reft upon the fame firm bafis. I (liali 

 therefore briefly mention the authors who agree with me 

 in opinion. 



Dr. Prieflley concludes from his own experiments, 

 that the difference of the air in different places, fuch as 

 is indicated by a mixture of nitrous air, is in general very 

 inconfiderable. He mentions that the air of Harthill 

 near Manchefter and that of Wiltlliire were about the 

 fame. 



The compilers of the Encyclopaedia fay ; " that the 

 general mafs remains upon all occafions pretty much the 

 lame." And Scheele is much of the fame opinion. 



But the accurate Fontana fpeaks with more eonfidence, 

 and is more explicit. His affertions are founded upon 

 the refult of many experiments, and he is inclined to 

 believe, that the flight variations mentioned by fome 

 philofophers, are rather to be attributed " to the fallaci- 

 ous eflfedls of uncertain methods" than to any real dif- 

 ference in the air itfelf. He found the air of Iflington 

 and London to fuflFer an equal diminution from the mix- 

 Ture with nitrous air. The air taken at different heights 

 in London and Paris did not differ in purity. Air at the 

 height of 313 and 202 feet in London, differed fcarcely 

 at all ; and no difference was perceptible between the air 

 of thele heights and that of the ftreet adjoining. 



The more I refle(5l on this fubje£l, the more I am in- 

 clined to adopt the following fentiment of this laft men- 

 tioned 



