[25]This theory, though generally held till 1930, has apparently been disproved by Jeffries.
[26]The periods of revolution and rotation have since appeared not to be exactly the same.
[27]Radiometric measures of late years show the outer surface of Jupiter to be at a very low temperature.
[28]As these thickenings, which he called tores, were not perceived the next time the rings were seen edgewise—although probably there—it is needless to dwell more upon them.
[29]By continued, and quite recent, study at Flagstaff the content of this gas has been found to be for Jupiter and Saturn one half, for Uranus five times and for Neptune twenty-five times the amount of the atmosphere of the Earth.
A reader who seeks to know more of the later theories of the Solar System may find them in the book with that name by Russell, Dugan and Stewart.
[30]Since he wrote, the discovery of radio-active substances has given rise to a wholly new crop of theories about the early geologic processes in the Earth’s crust.
[31]It is now practically certain that a dark star would be of very high density and small size, which would make the warning before the catastrophe still shorter.
[32]The discussion was continued in the press, Percival’s main argument being in his article in the Astrophysical Journal for October, 1907. Among those who claimed that the canals were optical illusions was Mr. Douglass after his connection with the Observatory had ceased; although he had previously drawn many of them, and himself discovered those in the darker regions.
[33]In Popular Science Monthly, for September, 1907, Mr. Agassiz told his experience in observing at Flagstaff, and why the appearance of canals cannot be due to optical or visual illusions.