S30 COSMOS. 



to show the important influence exercised on vegetation and 

 agriculture, on the cultivation of fruit, and on the comfort ol 

 mankind, by differences in the distribution of the same mean 

 annual temperature, through the different seasons of the yeai. 



The lines which I have termed isochimenal and isoihercit 

 Alines of equal winter and equal summer temperature) are by 

 no means parallel with the isothermal lines, (lines of equal 

 annual temperature.) If, for instance, in countries where 

 myrtles grow wild, and the earth does not remain covered 

 with snow in the winter, the temperature of the summer and 

 autumn is barely sufficient to bring apples to perfect ripeness, 

 and if, again, we observe that the grape rarely attains the ripe- 

 ness necessary to convert it into wine, either in islands or in 

 the vicinity of the sea, even when cultivated on a western coast, 

 the reason must not be sought only in the low degree of sum- 

 mer heat, indicated, in littoral situations, by the thermometer 

 when suspended in the shade, but likewise in another cause 

 that has not hitherto been sufficiently considered, although it 

 exercises an active influence on many other phenomena, (as, 

 for instance, in the inflammation of a mixture of chlorine and 

 hydrogen,) namely, the difference between direct and diffused 

 light, or that which prevails when the sky is clear, and when 

 it is overcast by mist. I long since endeavoured to attract the 

 attention of physicists and physiologists* to this difference, 

 and to the unmeasured heat which is locally developed in the 

 living vegetable cell by the action of direct light. 



If, in forming a thermic scale of different kinds of cultiva- 



* " Heec de temperie aeris, qui terrain late circumfundit, ac in quo, 

 longe a solo, instrumenta nostra meteorologica suspensa habemus. Sed 

 alia est caloris vis, quern radii solis nullis nubibus velati, in foliis ipsis 

 et fructibus maturescentibus, magis minusve coloratis, gignunt, quem- 

 que, ut egregia demonstrant experimenta amicissimorum Gay-Lussacii et 

 Thenardi de combustione chlori et hydrogenis, ope thermometri metiri 

 nequis. Etenim locis planis et montanis, vento libe spirante, circumfusi 

 aeris temperies eadem esse potest coelo sudo vel nebuloso; ideoque 

 ex observationibus solis thermometricis, nullo adhibito Photometro, 

 baud cognosces, quam ob causam Galliae septentrionalis tractus Armo- 

 ricanus et Nervicus, versus littora, coelo temperate sed sole raro utentia, 

 Vitem fere non tolerant. Egent enim stirpes non solum caloris stimulo, 

 Bed et lucis, quse magis intensa locis excelsis quam planis, duplici 

 modo plantas movet, vi sua turn propria, tnm calorem in superficie 

 earum excitante." Humboldt, De distributione geographica plan- 

 tarum, 1817, p. 163-164. 



