VOL. XV.] PHILOSOPHICAL TKANSACTIONS. 1 63 



was before thought to be a light body, was but comparatively so, and had in- 

 deed a positive gravity, though less than that of other bodies, which we are con- 

 versant with. 



This notion was happily pursued by Torricellio, a successor of Galileo, who 

 rationally argued, that if the air's counterpoise was sufficient to raise and sustain 

 water at that height, and only to that height ; then must it be a just couh- 

 terpoise to a lighter liquor of a greater height ; but to a heavier liquor of a less 

 height. And making a trial in quicksilver, he found it to succeed accordingly; 

 and in a just proportion to the respective gravities of those fluids. And he has 

 by this means made the experiment, commonly called the Torricellian experi- 

 ment, much more manageable with quicksilver, in tubes of about 33 inches 

 English measure, than before vi'ith water, in much longer tubes. 



In pursuance of this notion, we find by several sorts of baroscopes, not only 

 that the air has gravity, but that it has a different gravity at different times and 

 places, according as its counterpoise is capable of sustaining quicksilver at differ- 

 ent heights; sometimes a little lower than 28 inches, sometimes a little higher 

 than 30 inches, and at other times at some middle height between these. 

 Which different weight of the air may reasonably be supposed, partly to pro- 

 ceed from, and partly, as Dr. Garden properly intimates, to cause the difference 

 in the weather and winds. 



That there is in our air a body, more subtile than the fumes and vapours 

 mixed with it in our lower region, and which with it make up that heterogene- 

 ous mixture which we commonly call air, seems to be very certain. But whether 

 that subtile body be, as Dr. Garden seems to suppose, much heavier than our 

 common air, I much doubt; and rather think it is not, not having hitherto had 

 any cogent experiment, either to prove it heavy or elastic. But it may, for 

 ought I know, be void as well of weight as spring; and what is found of either 

 in our common air, may be attributed to the other mixtures in it. 



The air being then of a different gravity at different times and places, it may 

 therefore be considered as a fluid, whose parts are in some places heavier, and 

 others lighter : and therefore much of a like nature as if they were different 

 fluids, of different specific intensive gravity one from the other. Now when 

 several fluids, or several parts of a fluid, are thus of different weights ; they 

 will, from the general nature of heavy fluids, when undisturbed, change places 

 with each other, till the heavier becomes lowest, and the lighter uppermost. 

 And this not only as to the minuter parts, as the sinking of sand, &c. but 

 much more as to larger parcels ; as when oil, wine, water, beer, or other the 

 like liquors, are put together in the same vessel. And the same must happen, 

 if some parts of the same liquor do accidentally acquire, by expansion or other- 



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