CLIMATES. 61 



tions as these, and others of the same kind, have 

 been made at various places, collected and re- 

 corded ; and in this way the surface of the earth 

 can be divided by boundary lines into various 

 strips, according to these physical differences. 

 Thus, the zones which take in all the places 

 having the same or nearly the same mean annual 

 temperature, have been called isothermal zones. 

 These zones run nearly parallel to the equator, 

 but not exactly, for, in Europe, they bend to the 

 north in going eastward. In the same manner, 

 the lines passing through all places which have 

 an equal temperature for the summer or the win- 

 ter half of the year, have been called respec- 

 tively isotheral and isochimal lines. These do not 

 coincide with the isothermal lines, for a place 

 may have the same temperature as another, 

 though its summer be hotter and its winter colder, 

 as is the case of Pekin compared with London. 

 In the same way we might conceive lines drawn 

 according to conditions depending on clouds, 

 rain, wind, and the like circumstances, if we had 

 observations enough to enable us to lay down 

 such lines. The course of vegetation depends 

 upon the combined influence of all such condi- 

 tions ; and the lines which bound the spread of 

 particular vegetable productions do not, in most 

 cases, coincide with any of the separate meteoro- 

 logical boundaries above spoken of. Thus the 

 northern limit of vineyards runs through France, 



