CH. XXXVII. HUMBOLDTS TRAVELS. 385 



language an immense number of facts about nature in all 

 parts of the world. 



His chief voyage was to America in 1799, when he spent 

 six years in Mexico, and along the shores of the Orinoco. 

 Here he began one of his greatest undertakings, namely 

 finding out the climate of different parts of the world, and 

 tracing out isothermal lines, or lines of equal heat over the 

 globe, showing what countries have the same average 

 temperature, and explaining why some enjoy an almost 

 equable climate all the year round, while others are very hot 

 in summer and cold in winter. For example, he pointed 

 out that Greenland is much colder than Lapland, even in 

 places which are on the same line of latitude, because a 

 cold current from the North Pole flows past Greenland, 

 while the warm Gulf Stream crosses over from the Gulf of 

 Mexico and washes the shores of Lapland. The import- 

 ance of this study of variations of temperature was first 

 pointed out by Humboldt, and it should be remembered 

 as one of his most original investigations. 



Again, in his long journeys through South America, he 

 traced everywhere the different species of plants which 

 grew at various heights, even up to 20,000 feet on the slopes 

 of the Andes. This led him to try and find the reasons 

 why certain plants are only to be found in certain areas, in 

 the same way that Buffon had worked out the distribution of 

 animals. When he returned to Paris in 1804 he had col- 

 lected an immense number of facts as to the heights of 

 mountains, the climate of countries, the minerals and metals 

 found in them, the active and extinct volcanoes, the nature 

 of the rocks and soils, the vegetation and the animals j and 

 with the help of the best scientific men in Paris (each 



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