CLIMATE 49 



tion ; and owing to the prevalence of sunless days in autumn 

 the ripening process of the young wood of fruit-trees and 

 other plants is often but imperfectly done. 



Temperature. According to Dr Buchan's maps of the 

 temperature of the British Isles, the mean annual temperature 

 of the county of Argyll is 48 -5 F. ; the average variation 

 being 39-5-42* F. in January to 56-57'5 F. in July. 



In mid-winter (January) the only parts of Britain that have 

 a mean temperature of 41 F. and over are situated west of a 

 line drawn from Loch Roag in Lewis southward through Skye, 

 Ardnamurchan, and the Isles of Mull and Islay. Colonsay 

 lies west of this line, with, if it be produced farther south, 

 the Isle of Man and Anglesea, the western seaboards of Wales 

 and the extreme south-western counties of England. Along 

 the East Coast, on the other hand, the isotherms of 38 and 

 39 are dominant. In mid-summer, again, we find that, owing 

 to the tempering influence of the Atlantic, the temperature 

 on the West Coast is generally lower than it is on the East. 



Elevation and Temperature. For every 300 feet ascent 

 that we make the thermometer drops 1 F. To get the 

 same decrease of temperature at sea-level we have to travel 

 more than a degree of latitude due north. Owing to this 

 natural decrease of temperature from south to north, we find 

 that the limit at which cultivation can be carried on gradually 

 descends from an altitude of 2000 feet in the south of Eng- 

 land to sea-level in the Shetland Isles. The bracken is said 

 to determine the line of cultivation in Britain, but on the 

 West Coast it is not uncommonly found growing at elevations 

 at which few crops could be grown with profit. In a low- 

 lying island like Colonsay, the difference in temperature 

 between sea-level and the highest point is so small (less than 

 2 F.) as to be barely noticeable, and hardly sufficient to affect 

 the distribution of plants. On the mainland, however, and 

 wherever the land attains to a considerable altitude, we find, 



4 



