t 417 ] 



LXXlV. Thoughts on Atmospheric Density and Pressure. 

 By Thomas Charlton Speer, Esq. 



To Mr. Til loch, — Sir, 

 -LNotwith standing the wide field of inquiry which the 

 mechanical history of the atmosphere presents to us, and 

 notwithstanding the interest which such inquiry must na- 

 turally excite in a philosophic mind, yet we must confess, 

 our labours in it have been but partial, and our knowledge 

 of it is but limited : — the attention of men of science has 

 (comparatively speaking) been mostly confined to its che- 

 mical history, probably from its more immediate connection 

 with, and relation to, practical and useful results. On the 

 former subject, therefore, I beg to offer some ideas' which 

 have suggested themselves to me, and which relate to one of 

 its principal (though I think least understood) properties. 



Unaided by the lights of natural philosophy or the force 

 of experiment, it scarcely comes within the limits of human 

 conception, that that invisible inodorous aeriform mass of 

 fluid surrounding our globe should at all be subject to the 

 laws or possess the properties of matter; but particularly 

 that it should possess either density, weight, or pressure. 



These three properties of atmospheric air, viz., density, 

 weighty and pressure, are often misunderstood, and gene- 

 rally confounded with each other, particularly the two 

 former. Now we well know that attraction of cohesion and 

 attraction of gravitation sensibly differ from each other; if 

 not, platina would be the hardest, and the diamond the 

 heaviest, bodies in nature. We know that their action is 

 quite "different, that of the one being inversely as the squares of 

 the distances, that of the other increasing at a much quicker 

 rate as bodies approach* the smaller the distance the greater 

 its power. By the word density therefore, (which in parti- 

 cular seems often misapplied and misunderstood.) I mean, 

 strictly speaking, impermeability, or that power in a body 

 by which it is enabled to resist or obstruct (more of less) the 

 passage of other bodies through it, and which may be esti- 

 mated by the greater or less difficulty with which such re- 

 sistance or obstruction is conquered. Now this impenne- 

 Vol. 33, No>r 134, June 1809. Dd ability 



