7 



1° C. is in the mean 540 F. feet = 90 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 

 summits, the height becomes rather less (510 feet for the 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 than 

 they do at lower stations. 



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

 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 comparatively 

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

 near them, and circulation produced, especially in the horizontal 

 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 

 summer than in winter, amounting for example to 1° C. for MO feet 

 in July, and for 710 feet in January. The cause of this is, that in 

 the lower parts of the mountains the differences between single 

 months are greater than in the higher parts. 



11. The elevation of the point, near which on a vertical line the 

 depression of temperature is the greatest, is a different one in every 

 month. It is the highest, when the Alps are covered with snow, in 

 December and January ; from March to September this point is 

 generally found near the limit of snow ; in October and November 

 it lies below the snow-line. 



12. The height of the snow-line in the months does not coincide 

 always with the same isothermal. In January the snow- line and 

 the isothermal of 0° are both nearly on the base of the Alps ; but 

 from this time to July and August, the isothermal of zero moves 



