S8 M. Humboldt on Isothermal Lines^ 
winter and the summer should not exceed 1°.8, or nearly a 
quarter of the difference between the mean temperature of the 
winters or the summers of Montpellier and Paris. In speak- 
ing of the limits of the cultivation of plants upon mountains,^ I 
shall explain the true cause of this apparent contradiction. In 
the mean time, it may be sufficient to remark, that our meteo- 
rological instruments do not indicate the quantity of heat, 
which, in a clear and dry state of the air, the direct light pro- 
duces in the more or less coloured parenchyma of the leaves 
and fruits. In the same mean temperature of the atmosphere, 
the developement of vegetation is retarded or accelerated, ac- 
cording as the sky is foggy or serene, and according as the surface 
of the earth receives only a diffuse light, during entire weeks, 
or is struck by the direct rays of the sun. On the state of 
the atmosphere, and the degree of the extinction of light, de- 
pend, in a great measure, those . phenomena of vegetable life, 
the contrasts of which surprise us in islands, in the interior of 
continents, in plains, and on the summit of mountains. If we 
neglect these photometrical considerations, and do not appre- 
ciate the production of heat in the interior of bodies, and the 
effect of nocturnal radiation in a clear or a cloudy sky, we shall 
have some difficulty in discovering, from the numerical ratios 
of the observed summer and winter temperatures of Paris and 
London, the causes of the striking difference which appears in 
France and England in the culture of the vine, the peach, and 
other fruit-trees 
When we study the organic life of plants and animals, we 
must examine all the stimuli or external agents which modify 
their vital actions. The ratios of the mean temperatures of the 
months are not sufficient to characterise the climate. Its in- 
fluence combines the simultaneous action of all physical causes; 
and it depends on heat, humidity, light, the electrical tension of 
vapours, and the variable pressure of the atmosphere. It is the 
last cause which, on the tops of mountains, modifies the perspi- 
ration of plants, and even increases the exhaling organs. In 
making known the empirical laws of the distribution of heat 
** Young’s Travels in France^ vol. ii. p, 195. 
