120 The Climate of Great Britain, 



ness of tlie British climate — for our warm winters and cold 

 summers. It will appear, on examination, that nearly all the 

 constant causes affecting the temperature of a climate operate 

 to raise the mean temperature of our year, while, of variable 

 causes those which tend to generate increased heat operate 

 in winter, while those which have a contrary effect operate in 

 summer. 



Humboldt enumerates among the causes tending to exalt 

 temperature, the following non- variables : — The vicinity of a 

 west coast in the northern temperate zone ; the configuration 

 of a country cut up by numerous deep bays and far-penetrat- 

 ing arms of the sea ; the right position of a portion of the dry 

 land, i.e., its relation to an ocean free of ice, extending beyond 

 the polar circle, or to a continent of considerable extent which 

 lies beyond the same meridional lines under the equator, or 

 at least in part within the tropics ; the rarity of swamps 

 which continue covered with ice through the spring, or even 

 into summer ; the absence of forests on a dry, sandy soil ; 

 and the neighbourhood of an ocean-current of a higher tem- 

 perature than that of the surrounding sea. 



All these causes, it will be observed — except the neighbour- 

 hood of a tropical continent on the same meridian — tend to in- 

 crease the mean heat of the climate in England. The great Gulf 

 Stream probably exercises a more important influence than 

 any of the others. Its position is indicated in Figs. 1 and 2. 

 Humboldt attaches a high importance to the presence of a 

 tropical continent on the same meridian ; and he considers that 

 the climate of Europe is warmer than that of Asia, because 

 Africa, with its extensive heat-radiating deserts, lies to the 

 south of Europe, while the Indian Ocean lies to the south of 

 Asia. There are objections, however, to the reasoning he 

 adopts. In the first place, if the heat-radiating power of a 

 continent really influenced the country lying to the north, it 

 should tend to lower rather than raise the temperature, for the 

 ascending currents of air would strengthen the currents of 

 colder air pouring in from the north, and these currents — 

 on Humboldt's assumption that the country directly to the 

 north is that affected — would lower the mean annual tempera- 

 ture. It would only be exceptionally that the warmer 

 returning currents would descend, and thus exalt the tempera- 

 ture. It seems clear, however, that Asia is the continent 

 "hiefly affected by the heat-radiating power of Africa ; since 

 ohe cold currents from the north travel eastwards, while the 

 warm return-current has a westerly motion. We should thus 

 attribute the milder climate of Europe rather to the influence 

 of the tropical parts of the Atlantic Ocean, than to the cause 

 assigned by Humboldt, and we should invert the effects he 



