394 TRANSACTIONS OF THE 



amounting frequently to subversions of the law of the decrease of temper- 

 ature with the height. This will perhaps be best explained by supposing 

 an extent of country diversified by plains, valleys, hills, and table lands to 

 be under atmospheric conditions favorable to rapid cooling by nocturnal 

 radiation. Each part being under the same meteorological conditions, it 

 is evident that terrestrial radiation will proceed over all at the same rate, 

 but the effects of radiation will be felt in different degrees and intensities 

 in different places. As the air is in contact with the declivities of hills 

 and rising grounds, becomes cooled by contact with the cooled surface, it 

 acquires greater density, and consequently flows down the slopes and 

 accumulates on the low lying grounds at their base. It follows, therefore, 

 that places on rising ground are never exposed to the full intensity of frosts 

 at night; and the higher they are situated relative to the immediately sur- 

 rounding districts the less are they exposed, since their relative elevation 

 provides a ready escape downwards for the cold air almost as speedily as 

 it is produced. On the other hand, valleys surrounded by hills and high 

 grounds not only retain their own cold of radiation, but also serve as reser- 

 voirs for the cold heavy air which pours down upon them from neighboring 

 heights. Hence mist is frequently formed in low situations, whilst adjoin- 

 ing eminences are clear. Along low lying situations in the valleys of the 

 Tweed and other rivers of Great Britain laurels, araucarias, and other trees 

 and shrubs were destroyed during the great frost of Christmas, 1860, 

 whereas the same species growing on relatively higher grounds escaped, 

 thus showing by incontestible proof the great and rapid increase of temper- 

 ature with height at places rising above the lower parts of the valleys. 



This highly interesting subject has been admirably elucidated by the 

 numerous meteorological stations of Switzerland. It is there observed in 

 calm weather in winter, when the ground becomes colder than the air 

 above it, that systems of descending currents of air set in over the whole 

 face of the country. The direction and force of these descending currents 

 follow the irregularities of the surface, and, like currents of water, they 

 tend to converge and unite in the valleys and gorges, down which they flow 

 like rivers in their beds. Since the place of these air-currents must be 

 taken by others, it follows that on such occasions the temperature of the 

 tops of mountains and high grounds is relatively high, because the counter 

 currents come from a great height, and are therefore warmer. Swiss vil- 

 lages are generally built on eminences rising out of the sides of the mount- 

 ains with ravines on both sides. They are thus admirably protected from 

 the extremes of cold in winter, because the descending cold air-currents are 

 diverted aside into the ravines, and the counter currents are constantly 

 supplying warmer air from the higher regions of the atmosphere. 



Though the space filled by the down flowing current of cold air in the 

 bottom of a valley is of greater extent than the bed of a river, it is yet 

 only a difference of degree, the space being in all cases limited and well 

 defined, so that in rising above it in ascending the slope the increased 

 warmth is readily felt, and, as we have seen, in extreme frosts the destruc- 

 tion to trees and shrubs is seen rapidly to diminish. 



The gradual narrowing of a valley tends to a more rapid lowering of 

 the temperature, for the obvious reason that the valley thereby resembles a 

 basin almost closed, being thus a receptacle for the cold air-currents which 

 descend from all sides. The bitterly cold furious gusts of wind which are 

 often encountered in mountainous regions during night are simply the out- 

 rush of cold air from such basins. 



Two chief causes which tend to counteract these effects of terrestrial 

 radiation are forests and sheets of water. * * * Deep lakes may be 



