SUMMER TEMPERATURE. 41 



plants of Palestine. Florida, and the Carolinas 

 also, differ widely from each other, though dif- 

 fering but little in latitude. Isothermal lines 

 have, therefore, been constructed by naturalists, 

 that is, imaginary lines passing through those 

 places whose mean annual temperature is the 

 same. Thus, an isothermal line may be drawn 

 through Dublin, London, Manheim, in Ger- 

 many, and Vienna, in each of which the 

 average annual temperature is 50. But it has 

 been found by observation, that it by no means 

 follows that the summer is equally hot, and the 

 winter equally cold, at each of these places, for 

 it appears that an isothermal line passing from 

 west to east over the continent will, on an aver- 

 age, have a higher summer temperature and a 

 lower winter one, as it proceeds eastward. This 

 will be shown on the above line. 



Mean Mean Mean Differ- 



Annual Summer Winter ences. 



Temperature. Temperature. Temperature. 



Dublin ... 5Qo ... 59 ... 41 ... 18<> 



London ... 50 ... 63 ... 39 ... 24 



Manheim . . 50 ... 67 ... 33 ... 34 



Vienna ... 50 ... 69 ... 32 ... 37 



Thus, Vienna, though far hotter in summer 

 than Dublin, has a much colder winter. The 

 amount of average summer temperature is, 

 however, of more importance to the range of 

 some plants than the mean annual temperature, 

 inasmuch as they flourish and ripen their seeds 

 before the cold of winter conies on. This is 

 singularly exemplified at the Land's-end, in 

 England. Here the summer temperature is so 

 low, that the vine, apricot, and greengage will 

 4* 



