XXXU INTEODUCTION. 



Mdeorology, pp. 97-100). The mean temperature of tlie northern parts 

 of the county, where the ground rises to 400 feet or more, woul'd be on 

 that account rather more than a degree colder than that of the county 

 generally. 



It is well known that the character of vegetation alters both 

 with altitude and latitude ; and, since the mean temperature of a 

 place is influenced by both of these, some connection may be inferred 

 between its vegetation and its mean temperature. Mr. Watson, in the 

 Ct/bele Britannica, has divided the surface of the island into six zones, 

 each covering a range of about 3° of mean annual temperature, and 

 having a flora marked by certain peculiarities. All south-east England, 

 including Middlesex, belongs to the inferagrarian zone, of which Cle- 

 matis Vitalba is characteristic. The annual range of temperature, 

 however, exerts a more direct influence on the flora of a place. The 

 summer heat may be sufficient to satisfy the needs of plants which are 

 either annuals or send up annual stems, where the climate is extreme 

 and the mean temperature low ; while their existence may not be pos- 

 sible in a place with a higher mean temperature but a more equable 

 climate. Thus, Mr. Baker remarks that ' on the Andes, where the mean 

 temperature is nearly the same all the year round, they cannot grow 

 grain much above 7,000 feet, where the annual mean is 55°. In Britain 

 we have to stop at about 44°, and in Switzerland they stop at 40° ; but 

 in Norway wheat goes up to the 64th, oats to the 65th, rye to the 

 67th, and barley to the 70th parallel of latitude, where the annual 

 mean temperature is 32° or a little less. They can grow grain in 

 places at least 12° lower in mean temperature than we can in England, 

 and to get in Europe a mean of 55° we should have to go to Madrid 

 or Milan ' (Flvra of Northimiherland and Durham, p. 56). 



The great characteristic of British climate, as compared with other 

 European countries in the same latitude, is the comparatively small 

 range of temperature between winter and summer. This absence of 

 extremes either of heat or cold is due to the influence of the sea, and 

 distinguishes an insular from a continental climate. Mr. Glaisher has 

 deduced from registers kept from 1771 to 1849 the following mean 

 temperatures for the seasons at Greenwich (Phil. Trans., 1850, 

 p. 594) :— 



The difference between winter and summer is only 22-4°. Compare 

 this with the range at Munich, 34-5° j at Madrid, 33-5°; and at 



