WEIGHT or THE AIE. 229 



of the weight, the aerial particles which may still be in space must 

 forcibly escape from the terrestrial orbit. Perhaps it is, in fact, in 

 these elevated regions at the very limits of the spheres of attraction 

 of the heavenly bodies that the exchange of their gaseous particles 

 takes place. However that may be, it is at a height very inconsider- 

 able in comparison with the extreme limit indicated by Laplace, that 

 the atmosphere ceases to be respirable by man. At the summit of 

 Etna, that is to say, at an elevation of two miles, we have nearly a 

 third of the aerial mass under our feet. At 3 J miles, a height above 

 which a great many mountains raise their peaks, the column of air 

 which rests on the ground has already lost one half of its weight ; 

 consequently, all the gaseous mass which extends far into the sky to 

 immeasurable distances, is simply equal to the aerial strata com- 

 pressed into this lower region. 



More than two hundred years ago, Perier, following the indica- 

 tions of his brother-in-law Pascal, established by the first direct ex- 

 periment the diminution of the weight of the air in a vertical direction ; 

 he ascended Puy-de-D6me, with the barometer in his hand, and during 

 the ascent the column of mercury which measured the atmospheric 

 pressure never ceased to sink gradually in the tube, and thus the 

 means of measuring the height of mountains above the level of the 

 sea, by simply reading the barometrical indications, was discovered. 

 Since this epoch science has made great progress, the precise law of 

 the decrease of the weight of the air and all other elastic gases has 

 been brought to light by Mariotte, and innumerable travellers have 

 been able, with the aid of the barometer, to indicate approximately 

 the altitude of the salient points in the various countries that they 

 have traversed. Nevertheless, one can never be sure that the baro- 

 meter has furnished perfectly exact measures of height. In each, 

 barometic reading we must take into account the temperature, 

 the quantity of watery vapour contained in the atmosphere, the 

 agitation of the winds, in a word, all those physical conditions of the 

 air, whose weight we are about to measure, and each of these second- 

 ary observations makes a greater or less correction necessary in the 

 final result. The direct results obtained by trigonometry are at 

 present the only ones that give in an exact manner the height of 

 the surface. 



To ascertain the altitude of summits another means is also em- 

 ployed, which, in consequence of the defectiveness of the instruments, 

 generally gives results still less exact than those of the barometer. 

 This means consists in measuring the heat of boiling water. In. 



