resulting from Observations made in Eight Balloon- Ascents. 133 



stratum being no longer compressed, would dissipate itself in 

 its turn, and successively the whole atmosphere would finish 

 by evanescing. It is by cold that the extreme strata must lose 

 their elasticity and be checked from flying away. If the general 

 equation of dilatability of air, 



_ p (l + *fl) 

 p -p (I+«6 ) P °> 



which links the pressure p to the density p and to the tempera- 

 ture 9, held good throughout the whole atmosphere, the final 

 condition of no pressure would be given by the temperature 



0= — - = -461°-2 Fahrenheit. 

 a 



This is the temperature which is considered as the absolute zero 

 in the mechanical theory of heat. 



Between the temperature at the earth's surface and the limit- 

 ing temperature, which prevents the expansion of the external 

 layer of the atmosphere, we know by observation that the tem- 

 perature of air decreases with the elevation, excepting accidental 

 anomalies sometimes observed in the lower strata, most suscep- 

 tible of being modified by the contact of the soil. 



The law of this decrement, on ascending into the atmosphere, 

 can at present be known only by experience ; and the best means 

 of investigating it is by balloon-ascents. 



The most important observations on this head hitherto made 

 are Mr. Glaisher's, an account of which has lately appeared*. 



The general conclusion of these remarkable observations is, 

 that the temperature of the air does not decrease uniformly with 

 increase of elevation above the earth's surface, but in a ratio 

 diminishing with the height. 



Now the analytical formula universally employed for calcu- 

 lating heights from barometric observations (Laplace's formula) 

 supposes implicitly quite the reverse, viz. that the temperature 

 decreases in an accelerating ratio. Denoting the absolute tem- 

 peratures (above the absolute zero estimated at — 461 0, 2 F.) of 

 the extremities of a column of air by t and t, the height of the 

 column by x, Laplace's formula is grounded on the hypothesis 

 that 



x=b(t *-f) s 



b being a constant. According to this law, the increment of 



* An Account of Meteorological and Physical Observations in Eight 

 Balloon-Ascents, made at the request of the Committee." By James 

 Glaisher, F.R.S. (From the Report of the British Association for the 

 Advancement of Science for 1862.) 



