Chap. 9.] The Barometer. 4*1 



fame which is now commonly known by the name of 

 a barometer *. 



Air acls upon barometers in two modes, by its 

 weight and by its elafticity. The variation, therefore, 

 of the preffure upon the refervoir is produced by two 

 caufes, by the variation in the weight of the incumbent 

 air, and by that of its elafticity. The weight of air 

 varies according to its denfity, and its intermixture 

 with other fubftances which are foluble by it j its elaf- 

 ticity varies according to its denfity, and the quantity 

 of heat with which it is charged. The greater pare 

 of foreign fubftances which intermix with air only, 



* '' To fill a barometer tube, (fays Mr. Adams) I take a clean 

 gjafs tube about thirty-three inches long, and pour quickfilver into 

 it by means of a fmall paper funnel; you obfcrve, that as the 

 quickfilver rifes in the tube, there are bubbles of air left behind in 

 ieveral parts : I continue pouring the quicklilver till it fills the tube 

 within about half an ipch of the top. I then apply my finger hard 

 and clofe upon the top of the tube, and invert it; by which means 

 the air that was on the top, now rifing through all the quickfilver, 

 gathers every bubble in its way. I revert the tube or turn it up 

 again, and the babble of air re-afcends, and if there are any fmall 

 bubbles left, carries them away; if, however, any remain, the opera- 

 tion muft be repealed. I now fill the tube to the top, and placing- 

 my finger on the open end of the tube, plunge that end into this 

 bafon of quickfilver; when the end of the tube is perfectly fub- 

 rnerged in the qaickfilver, I take my finger away, and you fee the 

 quickfilver remains fufpended in the tube, leaving a vacuum at top. 

 The column of quickfilver is about thirty inches in height; now 

 you will obferve that there can be no air in the fpace between the 

 quickfilver and the top of t>2 tube, for till the finger that clofed 

 the orifice in the bafon was taken away, that fpace was filled with 

 quickfiiver, and the quickfilver, which was thirty-three inches high, 

 funk in the tube, and left that fpace free from air, for no air could 

 get into the tube, unlefs it could force its way through the quick- 

 lilver in the bafon, and the thirty inches in the tube; or penetrate 

 through the fealed end of the tube : but as neither of thofe can be 

 done, it follows, that in the part of the tube which the quickfilver 

 leaves, there muft be a vacuum." Adams' '; Lefiures. Vol. i, p. 32. 



E e 3 under 



