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92 REPORT—1848. 
warmer in proceeding from south to north, and at the north cape the south- 
east winds are the coldest. Both the Scandinavian Alps and the Rocky 
Mountains in America form dividing walls in respect to climate. 
In approaching the tropics the curves flatten; the isothermal of 16° Reau- 
mur, or 68° Fahrenheit, nearly coincides throughout its course with the 
tropic of Cancer, its concavities in Africa and Trans-Gangetic India, and 
its intervening convexity in Hindostan, being quite inconsiderable. 
The dividing isothermal between the northern and southern thermal hemi- 
spheres, 21° Reaumur, or 79°2 Fahrenheit, is only a simple line in the neigh- 
bourhood of the Gallapagos, but branches out beyond on either side so as to 
enclose a connected space of highest temperature, narrow in the Atlantic, 
but spreading out in width in South Amrica, the Indian Ocean, and Equa- 
torial Polynesia beyond Australia. Out of this space it is only exceptionally 
(as for example on the north coast of Australia) that we find temperatures of 
22° Reaumur or 814° Fahrenheit, but not forming any continuous line. The 
fact of the space of highest temperature advancing farthest into the south- 
ern hemisphere in the Indian Ocean, and of this being also the locality of 
highest absolute temperature, are the reasons of the north-east trade be- 
coming at this season a north-west monsoon. 
In the month of January the greatest difference of mean temperature com- 
prised between 70° north and 70° south latitude, is 54° Reaumur, or 121°°5 
Fahrenheit. The thermic equator falls everywhere, excepting in Columbia 
and in Guinea, in the southern hemisphere ; but between this line and the 
latitude of 70° south, there are only twenty-two isothermals, while between 
it and 70° north there are fifty-four such lines. 
The isothermals of the southern hemisphere have the peculiarity of being 
much more inflected in the torrid than in the temperate zone. Where the 
alternation of land and sea from east to west ceases, the causes of inflection 
are absent. Besides the different effect of radiation on a solid or a liquid 
base, the configuration of continents is also influential in other ways. On 
it depend the courses of marine currents, whose influence becomes ciearly 
apparent in the prosecution of such an examination as the present. In draw- 
ing the isothermal lines across the Ocean, they depend exclusively on the 
observations of atmospheric temperature, the numerous observations of the 
temperature of the sea never being taken into account. This distinction is 
imperative where the atmospheric isothermals are the’ objects of representa- 
tion, and where we aim expressly at obtaining as accurate a distinction 
between cause and effect as possible. 
The cooling influence of the polar current on the coast of Chili was dis- 
covered by M. de Humboldt ; its amount is not the same throughout the 
year, but it is unmistakeably sensible at all seasons. This causes the 
convex summits of the isothermals (which in the southern hemisphere mark 
the coldest localities) to be always on the western coast of America, and 
the concavities on the eastern coast. The reason of this persistency is to be 
found in the cold current in question not being a superficial one, but having, 
as it appears from soundings taken in the voyage of circumnavigation of the 
Venus, a depth of 5480 feet. ‘‘C’est une section considérable des mers 
polaires marchant majestueusement du Sud au Nord.” 
The great curvature of the isothermals in the Southern Atlantic, is shown 
by a comparison of the temperatures of Rio Janeiro, St. Helena, Ascension, 
Christiansburg, Cape Town, and the Isle of Bourbon. The character of the 
vegetation at St. Helena must for this reason differ materially from that of 
the New Hebrides. Even if we assume a greater decrease of temperature 
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