THE STORY OF NINETEENTH-CENTURY SCIENCE 



but where practical tests with the thermometer locate 

 them on our globe as it is. London, for example, lies in 

 the same latitude as the southern extremity of Hudson 

 Bay ; but the isotherm of London, as Humboldt outlines 

 it, passes through Cincinnati. 



Of course such deviations of climatic conditions be- 

 tween places in the same latitude had long been known. 

 As Humboldt himself observes, the earliest settlers of 

 America were astonished to find themselves subjected 

 to rigors of climate for which their European experience 

 had not at all prepared them. Moreover, sagacious 

 travellers, in particular Cook's companion on his second 

 voyage, young George Forster, had noted as a general 

 principle that the western borders of continents in tem- 

 perate regions are always warmer than corresponding 

 latitudes of their eastern borders ; and of course the 

 general truth of temperatures being milder in the vicin- 

 ity of the sea than in the interior of continents had long 

 been familiar. But Humboldt's isothermal lines for the 

 first time gave tangibility to these ideas, and made prac- 

 ticable a truly scientific study of comparative climatol- 

 ogy- 



In studying these lines, particularly as elaborated by 



further observations, it became clear that the}' are by 

 no means haphazard in arrangement, but are dependent 

 upon geographical conditions which in most'cases are not 

 difficult to determine. Humboldt himself pointed out 

 very clearly the main causes that tend to produce de- 

 viations from the average or, as Dove later on called 

 it. the normal temperature of any given latitude. For 

 example, the mean annual temperature of a region (re- 

 ferring mainly to the northern hemisphere) is raised by 

 the proximity of a western coast ; by a divided config- 



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