Page:Popular Science Monthly Volume 8.djvu/654

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THE POPULAR SCIENCE MONTHLY.

bly the faintest and most difficult objects to observe in the solar system. Indeed, it is not wholly certain that they have ever been seen save in the telescopes of Mr, Lassell (their discoverer), Lord Rosse, and by the Washington refractor, although there are several telescopes now mounted both in Europe and in America which are adequate for their observation.

The satellite of Neptune, too, is a very difficult object, and hence it is extremely gratifying to find so many measures of these satellites as Prof. Newcomb has obtained. The telescope was mounted in November, 1873. From that time to April, 1875, there were made:

31 observations of Oberon.
34 "" Titania.
10 "" Umbriel.
8 "" Ariel.
54 "" Neptune's satellite.

It must be remembered that Neptune was only observed from July to February, and Uranus from January to May.

From a consideration of all the measures of Uranus's satellites, the author assigns as the mass of that planet 1/22600 of the mass of the sun, and he estimates the probable error of the denominator of this fraction at 100, so that we may say that this mass is not less than 1/22700 and not more than 1/22500; that is, the mass is determined within less than 1/200 part of its value. To understand the nicety of such measurements as have been made, it must be remembered that any error in the measures of the distance of the satellite from the planet is shown in the resulting mass of the planet in an amount not proportional to this error directly, but to the third power of the error.

The times of revolution of the satellites have been determined with high accuracy by a comparison of Newcomb's observations with those of the elder Herschel—the uncertainty in the period of Titania 8days.705897, being not more than one second of time, or 1/1000000 of the whole amount.

From the relative brightness of the satellites of Uranus, Prof. Newcomb concludes that they have masses not more than 1/60000 of that of Uranus itself, i. e., vastly less than the mass of our own moon.

It is an interesting fact too that the author suspects that the nearest of the satellites of Uranus (Ariel) "belongs to that class of satellites of which the brilliancy is variable, and depends on its position in the orbit." With regard to the interesting question as to the number of satellites of Uranus, Prof. Newcomb's testimony is as follows:

"No systematic search for new satellites of this planet was entered upon, partly because the season in which Uranus is in opposition is now an unfavorable one for prosecuting such a search, and partly because the attempt would have absorbed so much of the observer's time and energies as to detract from the excellence of the micrometer-observations. When faint objects, which might have been new satellites, were seen around the planet, their positions relative to the latter were noted; but in no instance was any such object found to accompany the planet. I think I may say, with considerable certainty, that there is no satellite within 2' of the planet, and outside of Oberon, having one-third the brilliancy of the latter, and therefore that none of Sir William Herschel's supposed outer satellites can have any real existence. The distances of the four known satellites increase in so regular a way that it can hardly be supposed that any others exist between them. Of what may be inside of Ariel, it is impossible to speak with certainty, since, in the state of atmosphere which prevails during our winter, all the satellites would disappear at 10' distance from the planet."

The second section of the memoir deals with the Neptunian system. Three principal determinations of the mass of Neptune have been made:

Bond's, which gives the mass 1/194000.
Struve's, """" 1/14446.
Lassell's, """" 1/17135.

From the work of the Washington telescope the mass results 1/19380, which agrees most remarkably with Bond's previous determination.

No evidence for an elliptic form to the orbits of any of these satellites has been made out: "We are thus led to the remarkable conclusion that the orbits of all the satellites of the two outer planets are less eccentric than those of the planets of our system, and that, so far as observations have yet shown, they may be perfect circles. No trace of a second satellite of Neptune has ever been seen, though several times carefully looked for, under the finest atmospheric conditions, during July, 1874."

We have thus far spoken mainly of the