APPENDIX. 391 



to and from surrounding objects. This remark applies especially 

 to the observations at the St. Bernard, which lies at a consider- 

 able distance from Geneva, and where the temperature is 

 unduly depressed by surrounding masses of snow. I do not, 

 however, attach much importance to these sources of error ; 

 and I have no doubt that under the most favourable conditions 

 the discrepancy shown by Riihlm.ann will be found to a greater 

 or less extent, but I differ from that writer in the inference that 

 he has drawn from the facts. 



If I have not misunderstood his remarks, Dr. Riihlmann 

 concludes that the true temperature of the successive strata of 

 air in the zone between the base and the summit of a mountain 

 is but slightly affected by the diurnal changes that are exhibited 

 in the range of the thermometer, and to a moderate extent only 

 by the changes of season as shown by the range of the monj:hly 

 means. He has not adverted to the fact that the differences 

 disclosed in his tables may be the result of changes in the rate 

 of decrement of temperature in ascending from the lower to the 

 higher station. He shows that, on the mean of the July obser- 

 vations, the mean temperature of the air between the levels of 

 Geneva and the St. Bernard is lower than the mean difference 

 of the temperatures observed at those places by 1-57° C. But 

 this is not inconsistent with the supposition that the thermo- 

 meters have recorded the true air temperature at each station, 

 but that the rate of decrement of temperature in ascending, ai 

 that season, diminishes rapidly in the successive vertical zones. 

 In the same manner the fact that the true mean temperature in 

 January is higher than the mean of the observed thermometers 

 by 1-83° C., might be accounted for by supposing that in winter 

 the rate of decrement is smaller in the lower strata, and increases 

 in ascending above the surface. It is equally true that, in both 

 cases, the facts may be consistent with such an irregular dis- 

 tribution of the atmosphere in successive layers, or strata, of 

 very unequal temperature as was apparent in most of Mr. 

 Glaisher's balloon ascents. What is completely proved is that 

 it is only under exceptional conditions that the hypothesis of an 

 uniform rate of decrement of temperature, directly proportional 

 to height above the sea-level, is approximately correct for 

 observations in the temperate zone, where there is a considerable 

 diurnal and annual range of the thermometer. 



