APPENDIX. 377 



as the height increases. Above the height specified the 

 observations prove that in each ascent the balloon passed 

 through successive strata of air whose temperature varied in a 

 completely irregular manner, the fall of temperature being 

 sometimes very rapid for an ascent of a few hundred feet, and 

 sometimes very slight in a much longer interval. In each of 

 the higher ascents we even find instances in which the thermo- 

 meter rose in ascending from a lower to a higher station, 

 reversing the ordinary progression. These alternations occurred 

 at various heights from 5000 to 25,000 or 26,000 feet above the 

 sea-level.* It seems to me very doubtful whether any safe con- 

 clusions can be drawn from averages deduced from separate 

 series of observations so discordant, but, in any case, I may 

 confidently assert that the results of actual observations do not 

 bear out the conclusions deduced by Mr. Glaisher. 



I desire further to point out that these balloon ascents were 

 all executed by day, in summer, and in weather as serene as 

 can ordinarily be found in our climate. If they did authorize 

 us to derive from them an empirical law regulating the vertical 

 distribution of temperature, this might, at the best, serve to 

 approximate to the true amount of atmospheric refraction found 

 by day in geodetical observations, but would be no guide to the 

 conditions obtaining by night, which are those important to 

 the astronomer. 



Mr. Glaisher has not failed to notice the great difference 

 shown by the observations made when the sky was overclouded 

 as compared with those under a clear or partially clear sky, and 

 has given a table showing that the mean results up to a height 

 of 4000 feet above the sea show a nearly uniform decline of i ^ 

 Fahr. for each 244 feet at ascent. The numerical results of 

 observations made under, or amidst, cloud appear to me of no 

 practical value, as they depend upon conditions which are 

 subject to constant variation. 



If it be true that observations in balloon ascents, which are 

 free from the disturbances caused by the vicinity of the earth's 



* Air nearly saturated with vapour is lighter than air relatively dry ; and 

 hence it may happen that, when a current of moist air meets one relatively 

 dry, it will flow over the latter if they are nearly at the same tempera- 

 ture, but if the drier current be much warmer, it may flow beneath it. 



