credit for showing that it is a consequence of the existence of radiation pressure combined with the principles of thermodynamics is due to Bartoli and Boltzmann."] [Footnote 116: Quoted by Moulton in his Introduction to Astronomy.] [Footnote 117: Introduction to Astronomy.] [Footnote 118: The term billions, here and elsewhere, is used in the American sense, 10^{9}.] [Footnote 119: The assumed number of stars here is ten times as great as in the other parts of this line.] CHAPTER XV THE SUN'S JOURNEY THROUGH SPACE Having gained some idea of the nature of the electrical hypothesis of solar disturbances and of the possible effect of other bodies upon the sun's atmosphere, let us now compare the astronomical data with those of geology. Let us take up five chief points for which the geologist demands an explanation, and which any hypothesis must meet if it is to be permanently accepted. These are (1) the irregular intervals at which glacial periods occur; (2) the division of glacial periods into epochs separated sometimes by hundreds of thousands of years; (3) the length of glacial periods and epochs; (4) the occurrence of glacial stages and historic pulsations in the form of small climatic waves superposed upon the larger waves of glacial epochs; (5) the occurrence of climatic conditions much milder than those of today, not only in the middle portion of the great geological eras, but even in some of the recent inter-glacial epochs. 1. The irregular duration of the interval from one glacial epoch to another corresponds with the irregular distribution of the stars. If glaciation is indirectly due to stellar influences, the epochs might fall close together, or might be far apart. If the average interval were ten million years, one interval might be thirty million or more and the next only one or two hundred thousand. According to Schuchert, the known periods of glacial or semi-glacial climate have been approximately as follows: LIST OF GLACIAL PERIODS 1. Archeozoic. (1/4 of geological time or perhaps much more) No known glacial periods. 2. Proterozoic. (1/4 of geological time) a. Oldest known glacial period near base of Proterozoic in Canada. Evidence widely distributed. b. Indian glacial period; time unknown. c. African glacial period; time unknown. d. Glaciation near end of Proterozoic in Australia, Norway, and China. 3. Paleozoic. (1/4 of geological time) a. Late Ordovician(?). Local in Arctic Norway. b. Silurian. Local in Alaska. c. Early Devonian. Local in South Africa. d. Early Permian. World-wide and very severe. 4. Mesozoic and Cenozoic. (1/4 of geological time) a-b. None definitely determined during Mesozoic, although there appears to have been periods of cooling (a) in the late Triassic, and (b) in the late Cretacic, with at least local glaciation in early Eocene.