Chapter III

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[16]Their existence was proved, although the grain of the best plates is too coarse to distinguish between sharp lines and diffuse bands.

[17]While written in the third person the words are clearly his own.

[18]His determination of the Martian temperature has since been very closely verified.

[19]In a letter to Dr. V. M. Slipher on Oct. 4, 1902 he writes:

“There has come into my head a new way for detecting the spectral lines due to a planet’s own atmospheric absorption, and I beg you will apply it to Mars so soon as the Moon shall be in position to make a comparison spectrum.

“It is this. At quadrature of an exterior planet we are travelling toward that planet at the rate of 18.5 miles a second and we are carrying of course our own atmosphere with us. Our motion shortens all the wave-lengths sent us from the planet, including those which have suffered absorption in _its_ atmosphere. When the waves reach _our_ atmosphere those with a suitable wavelength are absorbed by it and these wave-lengths are unaffected by our motion since it is at rest as regards us. Even were the two atmospheres alike the absorbed wave-lengths reaching us would thus be different since the one set, the planet’s, have been shifted by our motion toward it while the other set, our own, are such as they would be at rest. We thus have a criterion for differentiating the two. And the difference should be perceptible in your photographs. For the shift of Jupiter’s lines due to rotation is such as 8. × 2. = 16 miles a second produces, which is less than 18.5 and about what you will get now.”

[20]So far as the shooting stars are concerned this opinion was based upon their velocities, which have since been found in many cases to be greater than was then supposed.

[21]Opic has recently shown that the sun’s effective domain is even larger.

[22]Later observations seem to show that Mercury’s periods of rotation and revolution are not the same, but nearly so.

[23]It now appears very improbable that these are real comet families.

[24]Recent results indicate that these are much smaller, and sometimes move faster, than was formerly believed.

[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.

[34]The Director’s house was commonly known as “The Baronial Mansion.”

[35]Memoirs of the Lowell Observatory, Vol. I, No. II.

[36]Bulletin No. 32.

[37]In a recent letter from the Observatory Mr. E. C. Slipher describes a great white spot that appeared on the equator of Saturn in 1933. It behaved as of hot matter flung up from the interior, and after two or three days spread itself towards the East in the direction of the planet’s rotation. His explanation is that the level from which this matter came is revolving faster than the atmospheric shell, the new material coming to the visible surface constantly more and more in advance of the original spot—a confirmation of Percival’s calculations.

[38]Vol. XIV, No. 1.

[39]“The Evolution of Worlds,” p. 118 and _seq._

[40]Adams, “Explanation of the Motion of Uranus,” 1846.

[41]Proc. Amer. Acad., Vol. 1, p. 64.

[42]Proc. Amer. Acad., Vol. 1, p. 65 _et seq._

[43]Proc. Amer. Acad., Vol. 1, p. 144.

[44]Proc. Amer. Acad., Vol. 1, p. 332.

[45]Observatory “Memoir on a Trans-Neptunian Planet.”

[46]Much of the following account is taken from “Searching Out Pluto” by Roger Lowell Putnam and Dr. V. M. Slipher in the _Scientific Monthly_ for June, 1932, by whose courtesy it is used.

[47]515 asteroids and 700 variable stars were there disclosed.

[48]After X had been discovered two very weak images of it were found on photographic plates made in 1915—the year he published his Memoir.

[49]This figure slightly changed for later observations is on the opposite page.

[50]Dr. A. C. D. Crommelin, the highest authority in England on such matters, had expressed the same conclusion; and the Royal Astronomical Society had cabled its felicitations on the discovery. Professor Russell’s latest views may be found in _infra_.

[51]The non-expert reader must remember that the mass and the size—still more the apparent size—are very different things, and the mass is the only one that could be found by calculation, for this alone affects the attraction, which at such a distance is quite independent of the density and hence of the size. Moreover, the apparent size depends also upon the extent to which the surface reflects the light of the sun—technically termed the planet’s albedo—a matter that has no relation to the perturbation of another body.

[52]“The Astronomical Romance of Pluto”—Professor A. O. Leuschner—Publications of _The Astronomical Society of the Pacific_, August, 1932.

[53]See page 181 _supra_.

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