APPENDIX. 379 



available source from which any indications of a law of distri- 

 bution can be gleaned. Balloon observations have hitherto, so 

 far as I know, been confined to a few places in Europe ; and, 

 even if the results were more conclusive than they have hitherto 

 been, we should not be entitled to infer that they held good for 

 all parts of the earth. In countries where the course of the 

 seasons is more uniform, and the direction and force of the 

 winds less inconstant, it might be expected that the distribution 

 of temperature would exhibit some nearer approach to uni- 

 formity ; and the possibility of making observations at mountain 

 stations by night might enable us to form some conjecture as 

 to a condition of the atmosphere very different from that which 

 obtains when the influence of the sun is present. 



It cannot be said that the observations hitherto made on 

 mountains have done as much as they might do, if properly 

 conducted, to contribute to our knowledge ; but a few leading 

 facts may be derived from them, and it is worth while to point 

 them out. 



The most important of these is, perhaps, the influence of 

 plateaux of elevated land in raising the temperature of the 

 adjacent air. This is established by observation in all parts of 

 the world, and it would appear that the rapid fall of temperature 

 in the strata near the surface which is found at or near the 

 level of the sea, is equally marked when we ascend from a 

 plateau to an isolated summit. Both these conclusions, how- 

 ever, apply only to observations made in the summer of 

 temperate regions, or in the warmer parts of the earth. Apart 

 from this effect of a relatively heated surface which appears to 

 extend above the surface to a height of about 1500 metres, or, 

 in round numbers, 5000 English feet, mountain observations 

 give but slight confirmation to the belief that the rate of de- 

 crease of temperature, in normal conditions of the atmosphere, 

 diminishes as the elevation increases. 



In endeavouring to use the available materials one difficulty 

 arises from the fact that, in comparing the temperature of the 

 upper with the lower stations, observers have rarely been supplied 

 with simultaneous observations at the lower station, or that, when 

 these have been available, the distance has been so great that 

 the results throw little light on the probable condition of a 

 vertical column of air near the higher station. In parts of the 



