(5) The form of the Earth's orbit, especially its eccentricity (Croll).
(6) The shape and extension of continents and oceans.
(7) The covering of the Earth's surface (vegetation).
(8) The direction of the oceanic and aerial currents.
(9) The position of the equinoxes.
De Marchi arrives at the conclusion that all these hypotheses must be rejected. On the other hand, he is of the opinion that a change in the transparency of the atmosphere would possibly give the desired effect. According to his calculations, “a lowering of this transparency would effect a lowering of the temperature on the whole Earth, slight in the equatorial regions, and increasing with the latitude into the seventieth parallel; nearer the poles again a little less. Further, this lowering would, in non-tropical regions, be less on the continents than on the ocean, and would diminish the annual variations of the temperature. This diminution of the air's transparency ought chiefly to be attributed to a greater quantity of aqueous vapor in the air, which would cause not only a direct cooling, but also copious precipitation of water and snow on the continents. The origin of this greater quantity of water-vapor is not easy to explain.” De Marchi has arrived at wholly other results than myself, because he has not sufficiently considered the important quality of selective absorption which is possessed by aqueous vapor. And further, he has forgotten that if aqueous vapor is supplied to the atmosphere, it will be condensed till the former condition is reached, if no other change has taken place. As we have seen, the mean relative humidity between the fortieth and sixtieth parallels on the Northern hemisphere is seventy-six percent. If, then, the mean temperature sank from its actual value +5.3 by 4°–5°C., i. e., to +1.3 or +0.3, and the aqueous vapor remained in the air, the relative humidity would increase to 101 or 105 per cent. This is, of course, impossible; for the relative humidity cannot exceed 100 per cent, in the free air. A fortiori, it is impossible to assume that the absolute humidity could have been greater than now in the glacial epoch.
As the hypothesis of Croll still seems to enjoy a certain favor with the English geologists, it may not be without interest to cite the utterance of De Marchi on this theory, which he, in accordance with its importance, has examined more in detail than the others. He says, and I entirely agree with him on this
