14 
and also of Saturn are receiving more light and heat than 
their northern poles and hemispheres. 
When, therefore, the tropics (by the movement of the 
earth's axis) extended to the polar regions, the sun would 
be one quarter of the year (whatever its length) always 
above the horizon in the northern portion of the northern 
hemisphere ; and even at midnight would be 14° above the 
horizon, and there would be no real night during the 
summer. Spring and autumn would be as at the present, 
containing days of equal length and equal nights, but winter 
would have been very severe, and hence vegetation would be 
dormant, the sun during one-fourth of the year being below 
the horizon ; and thus, during the summer months, the tro- 
pical flora would grow with wonderful rapidity, they would 
during the winter be frozen and entombed in snow of great 
depth, which on the return of spring would, by rapid thaws 
and land floods, be carried away into the adjoining lakes of 
fresh water and embedded as coal seams. 
It can also be proved that without some alteration in the 
direction of the earth's axis, sufhcient heat and light could not 
have been afforded to the arctic circle to enable that region to 
produce the carboniferous flora. For the mean temperature 
of different parallels of latitude is equal to the sine of twice 
their latitude, therefore the temperature now enjoyed 
between the latitudes 70° and 80° is only 35J° of Fahrenheit, 
while that of the equator is 84° ; but suppose the Earth had 
double its present heat, or that of Venus, then the heat would 
be raised for those latitudes to 71°, while Brongniart main- 
tains that the heat of our tropics, 84°, is insufficient. Sup- 
pose, also, that the earth enjoyed double the quantity of heat 
and light, with the poles in their present position, the arctic 
circle would still have only a summer of a few weeks' dura- 
tion, not at all adequate to the growth of the huge fossil flora 
found there, and therefore under any supposed conditions 
