A HISTORY OF SCIENCE 



temperate regions are always wanner than correspond- 

 ing 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 practicable a truly scientific study of compara- 

 tive climatology. 



In studying these lines, particularly as elaborated by 

 further observations, it became clear that they are by 

 no means haphazard in arrangement, but are depend- 

 ent 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 deviations 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 (referring mainly to the northern hemi- 

 sphere) is raised by the proximity of a western coast; 

 by a divided configuration of the continent into penin- 

 sulas ; by the existence of open seas to the north or of 

 radiating continental surfaces to the south; by moun- 

 tain ranges to shield from cold winds; by the infre- 

 quency of swamps to become congealed; by the ab- 

 sence of woods in a dry, sandy soil ; and by the serenity 

 of sky in the summer months and the vicinity of an 

 ocean current bringing water which is of a higher tem- 

 perature than that of the surrounding sea. 



Conditions opposite to these tend, of course, corre- 

 spondingly to lower the temperature. In a word, 

 Humboldt says the climatic distribution of heat de- 

 pends on the relative distribution of land and sea, and 



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