Astronomical Causes and Their Influence.
We will now consider the effect of astronomical causes. Dr. Croll has elaborately discussed the variations in solar exposure to which the two hemispheres of our planet have been and are subjected.[23]
The distinguished Astronomer Royal of the Dublin Observatory, Dr. Ball, has shown that 63% of the gross solar energy received by either hemisphere reaches it during summer and the remaining 37% during winter.[24] High authorities, both before and since these publications, have discussed various phases of these influences, as well as offered remarkable and unverifiable hypotheses regarding the temperature of space, solar energy, and the heat absorptive power of a solar envelope.[25] It is not necessary to attempt a discussion or elaboration of these views. Should the interpretation herein rendered be correct, it follows that variations in the distance from or degree of solar energy could not have directly affected the surface temperatures of the globe prior to the culmination of the Ice Age, and that only since that age could these slight variations have acted, except in a conservative way. It is unquestionable that for many years past the temperature of the northern hemisphere has risen more rapidly than the southern. This condition is proved not only by correct deductions from actual conditions and laws, but by observation. This is also recorded geologically by the greater removal of glacial conditions in the northern hemisphere—although in both this removal is yet in progress.
In a globe wrapped in a mass of vapor by reason of evaporation maintained at the surface by its own heat and condensed upon the outer surface of the spheroidal cloud envelope, it is immaterial so far as surface temperatures are concerned, to what degree of outside heat it may be subjected. The only possible effects of variations in the distance from, or intensity of the exterior heat source being to influence the duration of the interior supply and the distance therefrom at which cloud condensation takes place.
In a globe thus enshrouded the same order of surface temperatures would follow, whether revolving in the orbit of Venus or that of Neptune—the actual influences being the greater rate of loss in the remoter position, the more rapid succession of geological climates, and the greater time necessary for the removal of glacial conditions, and for the establishment of solar climatic control. Could the earth have been removed during the Archæan Age to the orbit of Jupiter without disturbing other conditions, no change could have occurred in the order of succeeding geological climates prior to the Ice Age. The rate of receipt of solar energy would have been in the ratio of (5.2)²:1; and the actual retardation of loss would have been in this ratio, as also the rate of establishment of solar climatic control; the crust would have cooled quicker, and therefore have been thinner and less stable than at present.
The observed movements in the cloud envelope of Jupiter present phenomena warranting the belief that his atmosphere is non-transcalent.[26] In this particular it resembles the clouded atmosphere of the earth; and indicates a condition analogous to that of the earth in pre-glacial ages. The smaller planets by reason of their lesser masses have lost their available resident heat. Their atmospheres have become cleared and are both translucent and transcalent. Their surfaces can be observed, and their volumes and densities calculated with a reasonable degree of exactness. In the cases of the larger planets observations are confined to the surface of their spheroidal cloud envelopes, and hence to these planets are ascribed volumes and densities varying abnormally from those whose actual volumes can be measured. The satellites of Jupiter possess much greater densities than that ascribed to the great planet—were it possible to measure the actual volume of his enshrouded mass this apparent anomaly would be in whole or in part removed.
Not knowing the surface temperatures, the exact composition of the atmospheres, nor the dimensions of the planetary masses, the distances to which the cloud envelopes of Jupiter, Saturn, Uranus and Neptune may be expanded, are yet matters of conjecture. Whatever is known of these planets corroborates the interpretation herein rendered of the record of the geological climates of the earth.[27]