INTRODUCTION. 



ix 



It is not very manifest at first sight why the temperature should 

 be lower in valleys ; but the best interpretation of this fact seems 

 to be that the air is in a more rapid circulation over the exposed and 

 more elevated ground ; and that, as it cools by radiation, it becomes 

 heavier and glides down the sloping ground and is immediately 

 replaced by air of a warmer temperature. Hence the thermometer 

 does not indicate so low a temp, as in the valley below, which not 

 only receives the heavier and colder air from above, but, being less 

 subject to currents, radiation can proceed undisturbed, and its effects 

 accumulate, till the difference between the temp, in the valley and 

 that on the adjacent higher ground may be very considerable. 

 Moreover, as moisture prevails in lower ground, as long as evapo- 

 ration goes on, there must be an additional tendency to keep the 

 air in the valleys constantly at a lower temp, than in the higher 

 ground. When, then, the temp, falls below 32°, the preceding 

 causes are probably quite sufficient to account for the excess of 

 cold in the valleys.* 



On grouping the Counties into Divisions. In the Reports 

 I have followed the Divisions of Mr. Symons in his British Rain- 

 fall ; but in order to ascertain the relative effects of the frost over 

 groups of counties, I have found it desirable to arrange them rather 

 differently and more in correspondence with the prevailing iso- 

 therms, which, as already stated, are regulated by the physical fea- 

 tures of the land. The counties only partially agree with these 

 latter, hence the best possible arrangement cannot give more than 

 approximate values. t Again, in estimating the amount of losses 



* As an illustration, Mr. Dines in a paper " On the Temperatures of Hill and 

 Valley" in the Jl. of the Met. Soc, 1872, pp. 100—102, compares Denbies near 

 Dorking, alt., 610 ft., with Cobham, 6 ms. dist. and 65 ft. above the sea. On the 

 morning of Dec. 31st, 1870, the min. temp, at Cobham was l°-2, but at Denbies, 

 14° ; and out of 43 occasions, when the temp, was below 25 Q at both places, 

 the av.upon the hill was 23 0, 3 against 18° -9 in the valley. See corroborative 

 remarks by Mr. A. Buchan, in Trans. Bot. Edin. Vol. xii. pp. 51, 52. 



Regarding this fact from a practical point of view, besides the obvious con- 

 clusion of not planting the more tender shrubs in low-lying localities, it must be 

 borne in mind that radiant heat accumulates in valleys ; consequently vegeta- 

 tion is stimulated by the extra heat and moisture, and is therefore in a worse 

 condition for withstanding an excess of cold, than on higher and drier ground. 



t Thus Nottinghamshire, though on the Eastern plain, feels the effect of the 

 adjacent high land of Derbyshire ; *hile Lincolnshire should be divided into 

 a warmer eastern, and a colder western half 5 &c> 



C 



