1875.] The History of our Earth. 313 
Mr. Croll repudiates. He considers that it “‘ does not duly 
take into account the prodigious influence exerted on climate 
by means of the heat conveyed from equatorial to temperate 
and polar regions by means of ocean currents.” Were it 
not for this heat he maintains that ‘‘ the thermal condition 
of the globe would be totally different from what it is at 
present,” and he declares, further, that “the effect of 
placing all the land along the Equator would be diametrically 
the opposite of that which Sir Charles supposes.” Into 
this argument it will be necessary for us to enter. The 
more land is placed at the Equator the more the possibility 
’ of conveying the sun’s heat from tropical regions by means 
of ocean-currents is reduced. Heat could, then, only be 
transferred to the colder regicns of the world by means of 
the upper currents of the trades, since the heat conveyed 
along the earth’s solid crust by conduction is obviously in- 
significant. But these upper currents, or “‘ anti-trades,” are 
not well adapted for conveying heat. Even at the Equator 
the upper currents are nowhere below the snow-line, and 
consequently they play in a region whose mean temperature 
is below the freezing-point. If warm they would, as a 
matter of course, raise the snow-line above their own level. 
The heat carried up by the ascending air-currents at the 
Equator is not transferred to higher latitudes to be there 
employed in warming the earth, but is thrown off into the 
cold regions of space above. The upward current is 
really one of the most effectual means which the earth pos- 
sesses of wasting the heat derived from the sun. Conse- 
quently, of all places, here ought to be collected the substance 
best adapted for preventing this dissipation of the earth’s 
heat into space. Now, of all known substances water 
seems to possess this quality to the greatest extent, and be- 
sides—being a fluid—it is adapted, by means of currents, to 
convey to every region of the earth the heat which it receives 
from the sun. 
Without denying the force of these considerations, it still 
appears to us that there are certain circumstances which 
speak in favour of Lyell’s supposition. The average tem- 
perature of Western and Southern Europe is admittedly 
higher, latitude for latitude, than that of Asia and North 
America. But Europe is precisely that portion of the globe 
which has in its proximity to the south the greatest amount 
of intertropical land,—to wit, the wide expanse of Africa,— 
and towards the north the least amount of frigid land. 
America becomes narrower as we approach the torrid zone, 
and widens out as we recede towards the pole. Again, hot 
