CLIMATOLOGY. 821 



We have just seen, that the lines which I have called 

 isochimenals and isotherals (or lines of equal icinter and 

 equal summer temperature), are by no means parallel with 

 the isothermals, or lines of equal annual temperature. If, 

 however, in countries where the myrtle grows wild, and the 

 snow does not continue on the ground during winter, tho 

 temperature of summer and autumn is barely sufficient to 

 ripen apples thoroughly, and if the vine (to produce drink- 

 able wine) avoids islands, and in almost all cases proximity 

 to coasts, the reason is by no means exclusively the low- 

 summer temperature of such situations, shewn by the 

 thermometer suspended in the shade: it is also to be 

 sought in a difference which has been hitherto but little 

 considered, although known to be most actively influential in 

 other classes of phenomena (for example, in the bursting into 

 flame of a mixture of hydrogen and chlorine), I mean the 

 difference between direct and diffused light ; or that which 

 prevails when the sky is clear, and when it is veiled by cloud 

 or mist. I long since ( 394 ) attempted to call the attention 

 of physicists and vegetable physiologists to this difference, 

 and to the heat, unmeasured by thermometers, which is 

 locally developed in the vegetable cells by the action of 

 direct light. 



If we form a thermic scale of different kinds of cultiva- 

 tion, ( 395 ) beginning with that which requires the hottest 

 climate, and proceeding successively from vanilla, cacoa, 

 spices, and cocoa nuts, to pine apples, sugar cane, coffee, fruit- 

 bearing date trees, cotton, citrons, olives, sweet chestnuts, and 

 vines producing drinkable wine, an exact consideration of 

 their various limits, both on plains and on the declivities of 

 mountains, will teach us that in this respect other climatic re- 



