AEROSTATICS. 251 



The heat of the atmosphere appears to decrease in a 

 less ratio than the distance increases. M. DE SAUS- 

 SURE found, that by ascending from Geneva to Cha- 

 mouni, a height of 347 toises, REAUMUR'S thermo- 

 meter fell 4. 2, and that on ascending from thence 

 to the top of Mont Blanc, 1941 toises, it fell 20.7 : 

 this gives 221 feet English for a diminution of 1 of 

 FAHRENHEIT in the first case, and 268 in the se- 

 cond. Nevertheless, from the accuracy which the 

 rule for barometrical measurement possesses, it may 

 be inferred, that the decrease of heat for the great- 

 est height which we can reach, is not far from uni- 

 form ; but that the rate for any particular case must 

 be determined by observation, though the average in 

 our climate may be stated at 1 for 270 feet of 

 perpendicular ascent. On this subject, see LA 

 GRANGE, Mem. de Berlin, 1772, p. 206., &c. He 

 thinks that the hypothesis of a uniform decrease of 

 heat is the most conformable to appearances. 



EULER, in a volume of the same Memoirs, for 1754, 

 p. 140., considers a harmonical progression as the 

 most probable. If the sole cause of the diminution 

 of temperature were distance from the earth, and 

 if it were admitted that there is no current of air 

 perpendicularly upwards, as there certainly is not, 

 the diminution of temperature would follow the in- 

 verse ratio of the distance from the centre of the 

 earth. Transactions of the Royal Society of Edin- 

 burgh, vol. vi. p. 365. 



Professor 



