THE ATMOSPHERE. 



Pascal therefore caused Torricelli's tube to be carried 

 top of a lofty mountain, called the Puy-de-d6me, in Auvergne, 

 and the height of the column to be correctly noted during the 

 ascent. It was found, in conformity with the principle announced 

 by Torricelli, that the column gradually diminished in height as 

 the elevation to which the instrument was carried increased. 

 The experiment being repeated upon a high tower in Paris with 

 like success, there no longer remained any doubt of the fact, that 

 the column of mercury in the tube, as well as the column of water 

 in common pumps is sustained, not by the force vulgarly called 

 suction, nor by Nature's abhorrence of a vacuum, but simply by the 

 weight of the incumbent air acting in one case on the surface of 

 the mercury, and in the other on the surface of the water in the 

 well, in which the pump terminates. 



12. The instrument which we have here described, as used in 

 the experiment of Torricelli, is nothing more than the common 

 barometer. 



The methods of constructing and mounting it so as to adapt it 

 for use, and the precautions necessary to ensure the certainty and 

 precision of its indications, will be explained in another number 

 of this series ; meanwhile it will be sufficient for the present to 

 assume generally, that the mercurial column E F, suspended in the 

 glass tube equilibrates with, and therefore measures the weight of 

 a corresponding column of the atmosphere, extending from the 

 surface, c F, of the mercury in the cistern indefinitely upwards. 



It has been shown,* that air in its usual state is 772^ times 

 lighter than water. But water being 13| times lighter than 

 mercury, it follows that air must be 13^ times 772| times lighter 

 than mercury. By multiplying 772| by 13|, we obtain 10429. 

 It follows, therefore, that 10429 cubic inches of air are equal in 

 weight to 1 cubic inch of mercury. 



13. If the atmosphere, in ascending from stratum to stratum, 

 had constantly the same density, so that each cubic inch of air, at 

 all heights, would have the same weight, we could at once deter- 

 mine its entire height by the preceding experiment ; for since a 

 column of mercury, 30 inches, or 2| feet high, has the same 

 weight as a column of air extending from the surface of the 

 ground to the top of the atmosphere, and since the weight of the 

 mercury, bulk for bulk, is 10429 times greater than that of air, 

 it is evident that the height of a column of air as heavy as the 

 column of mercury, must be 10429 times greater than that of the 

 column of mercury, and would therefore be 10429 times 



that is 26072 feet, or 5 miles very nearly. 



104 



Vol. ii., p. 3. 



