elastic and compressible; and about the same period 

 Mariotte proved its density was in proportion to the 

 weight with which it was compressed. The stratum of 

 the atmosphere nearest the surface of the earth supports 

 the weight of all above it, and is the densest ; each stra- 

 tum as we ascend becomes lighter or more rare, because 

 its elasticity is less checked by having a less weight 

 pressing from above. Pere Cotte deduced, that the 

 ratio of the decrease of its density was in geometrical 

 progression, if we take the heights in arithmetical 

 progression. Thus, if the density at 1 mile high was 1, 

 and that at four miles high |, then that at 7 miles high 

 would be , at 10 miles high , at 13 miles high y 1 ^, &c. ; 

 but this ratio is much disturbed by changes in the tem- 

 perature of the strata of the atmosphere at different ele- 

 vations. Heat expands the bulk of air, and forces it to 

 occupy a larger space ; 1000 volumes of air at 32 of 

 Fahrenheit become expanded into 1057*34 volumes at 

 60 ; thus heat is a cause of the unequal rise and fall 

 of the barometer through equal spaces. Sir George 

 Shuckburgh made numerous experiments upon the ef- 

 fects of temperature on the atmosphere ; and from his 

 labours we have a table, which shows in feet how much 

 the spaces passed through may vary from temperature 

 in a fall of T ^ of an inch of mercury, the barometer 

 standing at 30 inches ; and by means of his theorem for 

 its application, we are now enabled to ascertain the 

 heights of mountains by the barometer as correctly as 

 by geometrical measurement. 



There exists at all times in the atmosphere a certain 



