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 



