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’ 
Sxor, III.] CLIMATE IN ITS GEOLOGICAL RELATIONS. 27 
“The cause which above all others must tend to produce great 
changes of climate, is the deflection of great ocean currents. A high 
condition of eccentricity tends, we have seen, to produce an accumu- 
lation of snow and ice on the hemisphere whose winters occur in 
aphelion. ‘The accumulation of snow in turn tends to lower the 
summer temperature, cut off the sun’s rays, and retard the melting 
of the snow. In short, it tends to produce on that hemisphere a 
state of glaciation. Exactly opposite effects take place on the other 
hemisphere, which has its winter in perihelion. There the short- 
ness of the winters, combined with the high temperature arising 
from the nearness of the sun, tends to prevent the accumulation of 
snow. ‘The general result is that the one hemisphere is cooled and 
the other heated. This state of things now brings into play the 
agencies which lead to the deflection of the Gulf Stream and other 
great ocean currents. 
“ Owing to the great difference between the temperature of the 
equator and the poles, there is a constant flow of air from the poles 
to the equator. It is to this that the trade-winds owe their exist- 
ence. Now, as the strength of these winds will, as a general rule, 
depend upon the difference of temperature that may exist between 
the equator and higher latitudes, it follows that the trades on the 
cold hemisphere will be stronger than those on the warm. When 
the polar and temperate regions of the one hemisphere are covered 
to a large extent with snow and ice, the air, as we have just seen, 
is kept almost at the freezing-point during both summer and winter. 
The trades on that hemisphere will, of necessity, be exceedingly 
powerful; while on the other hemisphere, where there is compara- 
tively little snow or ice, and the air is warm, the trades will 
consequently be weak. Suppose now the northern hemisphere to 
be the cold one. The north-east trade-winds of this hemisphere will 
far exceed in strength the south-east trade-winds of the southern 
hemisphere. ‘The median line between the trades will consequently 
lie to a very considerable distance to the south of the equator. We 
have a good example of this at the present day. The difference of 
temperature between the two hemispheres at present is but trifling 
to what it would be in the case under consideration; yet we find 
that the south-east trades of the Atlantic blow with greater force 
than the north-east trades, sometimes extending to 10° or 15° N. lat., 
whereas the north-east trades seldom blow south of the equator. 
The effect of the northern trades blowing across the equator to a 
great distance will be to impel the warm water of the tropics over 
into the Southern Ocean. But this is not all; not only would 
the median line of the trades be shifted southwards, but the great 
equatorial currents of the globe would also be shifted southwards. 
“Let us now consider how this would affect the Gulf-stream. 
The South American continent is shaped somewhat in the form of a 
triangle, with one of its angular corners, called Cape St. Roque, 
pointing eastwards. The equatorial current of the Atlantic impinges 
