272 LIGHT SCIENCE FOR LEISURE HOURS. 



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-penetrating 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 temperature than that of the surrounding 

 sea. 



All these causes, it will be observed except the 

 neighbourhood of a tropical continent on the same 

 meridian tend to increase the mean heat of the climate 

 in England. The great Grulf Stream probably exer- 

 cises 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, how- 

 ever, to the reasoning he adopts. In the first place, if 



