the Changes of Isothermal Lines. 237 



as the readers of this Journal are well aware, to M. Humholdt. 

 This philosopher has traced in the northern hemisphere eight such 

 lines, five of them confined chiefly to the opposite shores of the 

 Atlantic, and three of them extending round about two-thirds of the 

 earth's surface. The evidence given by these lines, together with 

 the recent maps of monthly isothermal lines by Professor Dove, shew 

 the north-western parts of Europe to possess a much milder climate 

 than any other localities of corresponding latitude in the same 

 hemisphere. 



M. Humboldt found, that of two stations of equal latitude, the 

 one in Europe and the other in North America, the mean tempera- 

 ture in the former was 4*1 of Fahrenheit above the mean annual 

 temperature of the latter. The attempts which have been made to 

 explain the reason of this great elevation of temperature in Europe 

 have dwelt chiefly on the proximity of the Atlantic, and the in- 

 fluence of the gulf stream. In so far as the western shore of a 

 continent has been shewn to be warmer than the eastern, the 

 proximity of the Atlantic would be available to explain the superior 

 temperatures in Europe, compared with the United States or British 

 possessions in North America. But, if the observations which have 

 been made on the western shores of North America can be relied on, 

 the shore of that continent, bordered by the far-stretching Pacific, 

 has much lower temperatures than similar latitudes in Europe, north 

 latitude 46° being on the same isothermal line with London between 

 62° and 53°. Consequently, after deducting what is due to a western 

 sea-board, there still remains an excess of temperature in Europe to 

 be accounted for. The gulf stream which, after a course of about 

 4000 geographical miles, passes along the coast of the United 

 States to the banks of Newfoundland, where it begins to cross the 

 Atlantic to the shores of Norway, has been repeatedly urged as a 

 reason for the high temperatures of Europe ; if we look at the re- 

 gisters of temperature of places on the North American coast, near 

 the banks of Newfoundland, we find them but slightly elevated by 

 that cause, while on the coast of Norway, where the gulf stream can 

 have far less influence, the temperature for the latitude is very 

 great, hence another source of heat is required to account for the 

 elevated temperatures of north-western Europe. At a distance, 

 varying according to the localities from 1500 to 3000 geographical 

 miles, there is in the Sahara of Africa a magazine of heat, usually 

 considered as the greatest on the face of the globe, and composed of 

 heated air capable of travelling with facility at ten times the velocity 

 of oceanic currents of water. 



The air on the African desert has usually the same direction as 

 the trade-winds, namely, north-easterly, which is now admitted to 

 be explained by the reasons given by Halley, namely, the influence 

 of the sun rarifying the air at the equator, and the rotation of the 

 earth on its axis. The prevailing winds of the desert taking away 



