^ John Eliot — On the occasional Inversion of the Temperature [No. 1, 



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 villages are generally built on emi- 

 nences rising out of the sides of the mountains with ravines on both 

 Bides. 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 destruction 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 outrush of cold air from such basins." 



The most important recent contribution to the subject is a memoir on 

 "Mountain Meteorology" by Professor William Morris Davis, Harvard 

 College, Cambridge, U. S., in which he gives a summary of the 

 facts up to date. In this he points out that examples of inversion 

 of temperature relations are by no means rare in mountain districts 

 in Europe and America, and that they are most common in winter. 

 He quotes a monograph of Professor Hnnn's which states that the 

 inversion is best shewn in hill-enclosed valleys where the air stagnates 

 and is not replaced by air fi*om above. Such inversions, it is there 

 pointed out, are most frequent during the passage of areas of high 

 pressure or the prevalence of anti-cyclonic conditions. The unusual 

 warmth in the hill regions is shewn to be an effect of the compres- 

 sion of the descending air, whilst the cold in the valleys and low 

 ground is due to other causes, and takes place in spite of the descent of 

 air into it. A remarkable example in Europe of the inversion due to 

 the prevalence of anti-cyclonic conditions occurred in December 1879 



