286 PRESSURE IN THE AlK, 



The facts which I have advanced are, I trus*, 

 amply sufficient to show that there is a pro- 



The diminution of atmospheric pressure at different points 

 of elevation, is equally proved by the different degrees of de- 

 pression which mercury, in a torrecilian tnbe, is found to 

 undergo ; the experiment on this subject, which was first 

 suggested by DESCARTES, and put in execution by PERKIER, 

 although made for the express purpose of measuring the 

 weight of the air, proves, in a manner, sufficiently satis- 

 factory, the progressive diminution of expansive pressure, 

 which the atmosphere sustains from the bottom to the top, in 

 consequence of increased dilatation. Mons. Perrier, so long 

 ago as the year l64>6, filled two tubes, of an equal bore, with 

 mercury, and observed the height of it in both to be the same, 

 viz. 26-24 inches French, in the garden of the convent of the 

 Friars Minins^ situated in the lowest part of Clermont ; ort 

 leaving one of the barometers at the bottom of the garden, 

 and one of the fathers to observe it, he took the other one to 

 the Puy de domme, which was elevated near 500 French 

 fathoms above the garden, and found its height to be only 

 23 2-11 inches. On his return to the town, as he descended 

 he found, at 150 fathoms above the garden, that is 350 below 

 the upper station, that the mercury had risen to 25 inches ; and 

 on descending to the plain, he found that it rose to the same 

 level as the one which he had left in the garden, and which 

 had not varied the whole of the day. Thus a difference of 

 elevation of 3,000 French feet, had occasioned a depression 

 of 3 1-18 French inches: from which it maybe concluded, 

 not that 3 1-18 inches of mercury weigh as much as 3000 feet 

 of air ; or one-tenth of an inch of mercury, as much as 

 96 feet of air; but that the expansive power of air dimi- 

 nishes in proportion to its dilatation ; the mercury in the* 

 tube sinking in the same manner as it is found to do, whew 



