62 
HAROLD’S DISCUSSIONS. 
must have been warm over the entire globe. This is 
also suggested by the foliage, the fossils of which we find 
in coal-seams. At least three thousand kinds of tree- 
ferns have been described, and eighty per cent of these 
are found in what we now call the tropical regions. 
The tree-ferns of the present day are all in the warm 
climate. Some of the other vegetation must also have 
been tropical. We conclude, therefore, that Great 
Britain, the northern part of America, and even Sibe¬ 
ria, must have had a tropical or semitropical climate. 
The air must have contained a much larger pro¬ 
portion of carbon dioxid than it does now, for all 
the carbon in the coal must have been taken from 
the atmosphere by growing vegetation. The plants, 
which were very much alike, also suggest that the 
air contained a great deal of moisture. The carbon 
dioxid and water together in the air prevent the 
sun’s heat from radiating, very much like glass. 
Glass lets the sunlight through, but keeps the heat 
in, as any of us may find out from going to a green¬ 
house, or even to our own windows. Experiment 
was made on Mount Whitney with an enclosure cov¬ 
ered tightly by glass. The sun alone shining through 
the glass produced a temperature of 268° within, 
while the temperature in the shade outside was only 
58°. Kept warm and moist by the protecting atmos¬ 
phere, the earth was like a great hothouse where 
dense jungles of vegetation grew in luxuriance and 
stored up carbon, which ages after was destined to 
become so invaluable to modern civilization. 
We have thus briefiy mentioned the leading facts 
