The same objections would bear against the theory ably set forth by Mr. Sereno E. Bishop, of Honolulu, which, in substance, is that there may be considerable variability in the sun's emission of heat, owing to fluctuations in the rate of the shrinkage of its diameter, brought about by the unequal struggle between the diminishing amount of heat in the interior and the increasing force of the gravitation of its particles, and by the changes in the enveloping atmosphere of the sun, which, like an enswathing blanket, arrests a large portion of the radiant heat from the nucleus, and is itself evidently subject to violent movements, some of which seem to carry it down to the sun's interior. Unknown electrical forces, he thinks, may also combine to add an element of variability. These supposed changes may be compared to those which take place upon the surface of the earth when, at irregular intervals, immense sheets of lava, like those upon the Pacific coast of North America, are exuded in a comparatively brief time, to be succeeded by a long period of rest. The heat thus brought to the surface of the earth would add perceptibly to that radiated from it into space in ordinary times. Something similar to this upon the sun, it is thought, might produce effects perceptible upon the earth, and account for alternate periods of heat and cold. A fourth astronomical theory is that there has been a shifting of the earth's axis; that at the time of the Glacial period the north pole, instead of being where it now is, was somewhere in the region of central Greenland. This attractive theory has been thought worthy of attention by President T. C. Chamberlin and by Professor G. C. Comstock,[DS] but it likewise labours under a twofold difficulty: First, the shifting of the poles observed (450 feet per year) is too slight to have produced the changes within any reasonable time, and it is not likely to have been continuous for a long period. But still more fatal to the theory is the fact that the warm climate preceding the Glacial period seems to have extended towards the present north pole upon every side; a temperate flora having been found in the fossil plants of the Tertiary beds in Greenland and northern British America, as well as upon Nova Zembla and Spitzbergen. [Footnote DS: See papers by these gentlemen read at the meeting of the American Association for the Advancement of Science, in Washington, in August, 1891. Professor Comstock's paper appeared in the American Journal of Science for January, 1893.]