248 Royal Society, 



mean elevation of the difterent parts of the Alps. The isothermals 

 rise where the mean elevation is greater ; they sink at the borders 

 and on smaller groups. This convexity of the isothermal lines in the 

 centre of the Alps is still more considerable if we represent by them 

 the temperature of the earth, since the latter is still more intimately 

 connected with the mass of the mountains, with the insulation and 

 radiation of the rocky substance; whilst for the temperature of the 

 air, differences of that kind are more eliminated by its mobility. 



4'. The vertical distances of two isothermals are the greatest near 

 the base of the Alps, attain afterwards a minimum, and become in 

 the higher parts again a little greater. The position of the minimum 

 takes place in the Northern Alps and the group of ^ the St. Gothard 

 at nearly 6000 French feet ; in the central parts at nearly TOGO ; for 

 the group of Mont Blanc it seems to be at a still greater elevation. 



5. The height corresponding to a depression of temperature of 

 1^ C. is in the mean 510 F. feet = fiO toises= 166 metres, if we 

 compare the lowest stations of continued observation with the 

 highest stations; but if we consider the temperature of the highest 

 sianmits, the depression becomes a little greater (510 feet for th© 

 Central Alps). 



6. The mean temperature of the air seems to be, for the highest 

 summits, from —13° to — 15° C. 



7. At the greater elevations the temperature of single months is 

 generally altered in this manner: — The temperatures of February 

 and January, of August and July, differ less from each other thaa 

 they do at lower stations. 



8. The influence of the general form of the surface on the tera- 

 perature is particularly evident when we consider the mean tempera? 

 ture of the months. The valleys during the winter are in general^ 

 colder than the mountains, the cold air sinking down and being ac-</ 

 cumulated in them; during the summer they are coujparatively 

 warmer, the heat being reflected and radiated by the insulated masses 

 near them, and circulation produced, especially in the Jiorizontal 

 direction : their climate is therefore subject to greater extremes>^^ 

 though in the annual mean it scarcely differs from the Alps in general. 

 The declivities during the winter are comparatively warmer, since the 

 air near the surface, after sinking down in the valleys, is replaced 

 readily by less cold strata. During the summer, particularly in 

 southern exposures, and if the relative height above the bottom of 

 the valley is not great, they are also a little warmer, since then 

 they can partially be reached by the ascending current of air. But 

 this increase of temperature being smaller than that of winter, these 

 situations have a more constant climate than the valleys. The mean 

 temperature of the year on declivities, particularly with southern 

 exposures, is therefore a little higher than the mean of the Alps in 

 general. 



9. Summits and declivities, with an exposure to north and to north- 

 east, show also the character of a constant climate ; but the tem- 

 perature of summer is much lower, and consequently the annual 

 mean is also sensibly depressed. 



10. The depression of temperature with elevation is greater in 



