1890.] Relations between the Hills and Plains of Northern India. 3 



The following extract from an article on Climate in the Encyclo- 

 pcedia Britannica (written by A. Buchan, Esq.), I believe, fairly repre- 

 sents the opinion of English meteorologists on this subject: — 



" These results which only affect the mean daily temperature in 

 different seasons, and which are due exclusively to differences of ab- 

 solute height, though of the greatest possible practical importance, yet 

 leave untouched a whole field of climatological research — a field embra- 

 cing the mean temperature of different hours of the day at different 

 heights, for an explanation of which we must look to the physical con- 

 figuration of the earth's surface and to the nature of that surface, whether 

 rock, sand, black soil, or covered with vegetation. 



" Under this head by far the most important class of conditions are 

 those which result in extraordinary modifications, amounting frequently 

 to subversions of the law of the decrease of temperature 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 atmos- 

 pheric conditions favourable 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 in contact with the declivities of hills and 

 rising grounds becomes cooled by contact with the cooled surface, it ac- 

 quires greater density and consequently flows down the slopes and ac- 

 cumulates on the low-lying ground 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 relatively to the im- 

 mediately surrounding district the less are they exposed, since their re- 

 lative elevation provides a ready escape downwards for the cold air 

 almost as speedily as it is produced. On the other hand, valleys sur- 

 rounded by hills and high grounds not only retain their own cold of 

 radiation, but also serve as reservoirs for the cold heavy air which pours 

 down upon them from the neighbouring heights. Hence mist is fre- 

 quently formed in low situations whilst adjoining 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 I860, whereas the same 

 species growing on relatively higher grounds escaped, thus shewing by 

 incontestible proof the g'reat and rapid increase of tempei^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 ob- 

 served in calm weather in winter, when the ground becomes colder 



