resulting from Observations made in Eight Balloon- Ascents. 1 39 



For instance, if we take 



^ -/ = 50°-8Fahr., 



a decrease of temperature corresponding to a height of 16,000 

 feet (not far from that of Mont Blanc), the correction would be 

 197 feet (supposing, for the sake of illustration, t == 525°, 



6 = 4-2). For 



t -t = 70°, 



corresponding to a height of 30,000 feet, the correction is 541 

 feet. 



Whatever be the value of b in (1), provided it is positive, the 

 correction to be applied to Laplace's formula is always negative. 

 After Mr. Glaisher's observations it is impossible to suppose b 

 negative; therefore all the heights calculated in balloon-ascents 

 must be diminished. 



It is not certain that the same correction is to be applied to 

 the altitudes of mountains deduced from barometric observations ; 

 for we must admit that, as a general rule, the temperatures of 

 the extremities of a column of air remote from any ground, will 

 not be the same as that of the air at two stations on the flanks 

 of a mountain, on account of the variable mode in which the sur- 

 face of the latter is heated and imparts its temperature to the air 

 in contact. 



Simultaneous thermometrical observations made on the top of 

 mountains and in the air at the same level by means of captive 

 balloons would tend to elucidate that question. 



So far as observation has hitherto gone, the altitude of a sta- 

 tion on a mountain, as derived from barometric observations, varies 

 with the hour of the day. From about 1 p.m. till 3 in the morning 

 the altitude goes on diminishing ; and then it increases from that 

 hour up to the moment of the maximum, which is usually about 

 an hour after the sun passes the meridian of the place*. 



Observations of the same kind should be repeated in many 

 places varying in exposition and climate. It is desirable that 

 the attention of alpine travellers should be directed to this 

 subject. 



Much may be expected from the joint exertion of many well- 

 informed persons who, now that alpine clubs are spreading every- 

 where, frequent the higher regions of the Alps and other moun- 

 tain-districts. 



* See Prof. Piazzi Smyth, " Astronomical Experiment on the Peak of 

 Teneriffe," in the Philosophical Transactions of the Royal Society of 

 London, 1859. E. Plantamour, "Mesures Hypsometriques dans les Alpes," 

 Mem. de la Soc. de Physique et d'Histoire Naturelle de Geneve, 1860. 

 Charles Martins, " Sur l'influence de la distance et la correction horaire des 

 differences de Niveau," Archives Scientifiques de la Bibliotheque Universelle 

 de Geneve, 1860. 



