DENSITY OF THE AIR. 121 



down a tube closed at one end, condensing the air before it, until 

 it can absolutely be urged no farther, the included air resisting its 

 descent as effectually as any metal ; upon releasing the pressure 

 it again expands to its original bulk. The stratum of atmosphere 

 in immediate contact with the earth is subject to the entire press- 

 ure of the superincumbent mass, and is thus denser, i. e. has 

 more atoms contained in the same space than the stratum imme- 

 diately above, and the second stratum is denser than the third, 

 and so on. This decrease of density gives a limit to the extent of 

 the atmosphere. 



The ancients imagined that our atmosphere reached at least as 

 far as the moon, but the discovery of the weight and pressure of 

 the air destroyed at once this magnificent vision. Comparing 

 the length of the mercurial column with the density of the aerial 

 medium, it follows that if the atmosphere is a uniform fluid, it 

 cannot exceed the elevation of five miles. But the air being 

 very dilatable, the higher portions sustaining as we have shown 

 a diminished pressure, must swell upwards and occupy a propor- 

 tionally greater space. This property removes the boundary of the 

 atmosphere to a much greater elevation. A height of 42 miles 

 would indicate a rarefaction of a thousand times, for it has been 

 proved that the density decreases in a geometrical ratio, as the 

 height increases in an arithmetical ratio, thus : 



The elevation being in miles, |U|3|6|9| 12 j 15 |&c. 



[he density will be |l|||i|&|l-Hi|l-32|&c. 



The famous Kepler first proposed a method of determining the 

 height of the atmosphere by means of the twilight. After sunset 

 there appears in the western sky a bright illumination called twi- 

 light, which is caused by the sun's rays shining upon the higher, 

 regions of the air. This twilight fades when the sun has sunk 

 below the horizon a certain distance ; thus, let C be the position 

 of a spectator upon the earth, encompassed with its atmosphere, 

 and SAB the lowest ray of the sun after sunset, B C being the 

 horizon. The sun's rays would now illluminate the portion of 

 atmosphere between A and B, producing twilight visible to a spec- 

 tator at D, but not tp one at C. Now the twilight, or this last ray 



