APPENDIX. 371 



in connection with the physics of the globe, but has a direct 

 bearing on two practical applications of science. The observa- 

 tions of the astronomer and the surveyor require a knowledge 

 of the amount of atmospheric refraction, by which the apparent 

 positions of the heavenly bodies, or of distant terrestrial objects, 

 are made to differ from the true direction ; and to determine 

 accurately the amount of refraction we should know the 

 temperature of the successive strata of air intervening between 

 the observer and the object. In determining heights by means 

 of the barometer, or any other instrument for measuring the 

 pressure of the air, it is equally necessary for accuracy to know 

 the variations of temperature in the space between the higher 

 and the lower station. 



Three different opinions have prevailed among physicists as 

 to the law, or supposed law, of the rate of variation of tempera- 

 ture in ascending from the sea-level. The simplest supposition, 

 and the most convenient in practice, is that the fall of tempera- 

 ture is directly proportional to the height, and this has been 

 adopted in several physical treatises. In English works the 

 rate has been stated at a fall of 1° Fahr. for 300 feet of ascent, 

 and by French writers the not quite equivalent rate of 1° C. 

 for 170 metres has been adopted. The formula proposed by 

 Laplace for the determination of heights from barometric ob- 

 servations, which has been very generally adopted by travellers 

 and men of science, implicitly assumes that the rate of decrease 

 of temperature is more rapid as we ascend to the higher regions 

 than it is near the sea-level, and this opinion was explicitly 

 affirmed by Biot in his memoirs on atmospheric refraction. A 

 third hypothesis may be said to have originated when, in 1862, 

 Mr. Glaisher made his report of the results of the famous 

 balloon ascents effected by him and Mr. Coxwell,* and among 

 others exhibited a table showing the average decline of tempera- 

 ture corresponding to each successive thousand feet increase of 

 elevation from the sea-level to a height of 29,000 English feet. 



As Mr. Glaisher's tables showed a gradual decline in the rate 

 of fall of temperature with increasing height, they clearly did 

 not accord with the ordinary assumption of an uniform rate, 



* See Reports of the British Association for the Advancement of 

 Hcience ioT 1882, pp. 451-453. 



