vf tie common Air at Sea „ 365 
whereas the common air of the inn (as I found by trial 
at the time) was fomewhat of an inferior quality, though 
ftill remarkably pure ; for one meafure of it with one of 
nitrous occupied 097 in five repeated trials. 
As the difference in the quality of the fea air examined 
on the fpot in the mouth of the Thames, November 
3d, and that which I gathered in the middle of the 
fea in rainy and windy weather, was fo remarkable, I 
fufpedted the reafon of this difference to be, that the air, 
put to the teft November 3d, had been expofed during 
feveral days to the influence of the fea without any mix- 
ture of land air, as it had been remarkably calm all that 
time ; and that the air gathered on the fea in windy wea- 
ther was mixed with air driven from the land by the 
wind, and incorporated with the fea air. This fuf- 
picion was afterwards ftrengthened when I found the 
air gathered at the fea fhore, on the evening of Novem- 
ber 5th, near as good as that which I gathered on a fair 
day in the mouth of the Thames, November 3d ; for the 
wind being N.W. the air driven upon the coaft was to be 
confidered astmefea air, without any mixture of land air. 
But after I had made up my mind about the difference 
of the above related experiments, a doubt rofe in me about 
a circumftance to which this difference might have been 
owing, at leaft in fome meafure ; the circumftance I mean 
C c c 2 was 
