<5 
M. Hiiiriboldt on Isothermal Lmes^ and the 
coQtinents, variable in their extent, and whose detached parts 
dragged away by currents modify in a sensible manner the cli- 
mate of the temperate zone. 
In distinguishing, as has long been done, between the solar 
and the real climate^ we must not forget, that the local and mul- 
tiplied causes wliich modify the action of the sun upon a single 
point of the globe, aro themselves but secondary causes, the ef- 
fects of the motion which the sun produces in the atmosphere, 
and which are propagated to great distances. If we consider 
separately (and it will be useful to do this in a discussion pure- 
ly theoretical) the heat produced by the sun, the earth being 
supposed at rest and without an atmosphere, and the heat due 
to other causes regarded as disturbing ones, we shall find that 
this latter part of the total effect is not entirely foreign to the 
sun. The influence of small causes will scarcely disappear by 
taking the mean result of a great number of observations ; for 
this influence is not limited to a single region. By the mobility 
of the aerial ocean, it is propagated from one continent to ano- 
ther. Every where in the regions near the polar circles, the ri- 
gours of the winters are diminished by the admixture of the co- 
lumns of warm air, which, rising above the torrid zone, are 
carried towards the poles : Every where in the temperate zone, 
the frequent west winds modify the climate, by transporting 
the temperature of one latitude to another When we re- 
flect, besides, on the extent of seas, on the form and prolonga- 
tion of continents, either in the two hemispheres, or to the east 
and west of the meridians of Canton and of California, we shall 
perceive, that even if the number of observations on the mean 
temperature were infinite, the compensation weuld not take place. 
It is, then, from the theory alone that we must expect to deter- 
mine the distribution of heat o ver the globe, in so far as it depends 
on the immediate and instantaneous action of the sun. It does 
not Indicate the degrees of temperature expressed by the dilata- 
tion of the mercury in a thermometer, but the ratios betw een the 
mean annual heat at the equator, at the parallel of 45®, and un- 
der the polar circle ; and it determines the ratios between the 
solstitial and equinoctial heats in different zones. By comparing 
Raymond, ISIanm-c sv.r la Fonnule Baromct. p. 103 and 113. 
