636 



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 

 diflBcult object, and hence it is extremely 

 gratifying to find so many measures of 

 these satellites as Prof. Newcomb has ob- 

 tained. The telescope was mounted in 

 November, 18Y3. From that time to April, 

 1875, there were made : 



31 observations of Oberon. 

 84 " " Titania. 



10 " " Umbriel. 



8 " " Ariel. 



54 " " Jfepiune^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 meas- 

 ures of Uranus's satellites, the author as- 

 signs as the mass of that planet i^guij 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 -2-2-7-oxri ^"d i^ot 

 more than saiorr ; that is, the mass is de- 

 termined within less than -j^o 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 pro- 

 portional 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 observa- 

 tions with those of the elder Ilerschel 

 the uncertainty in the period of Titania =. 

 8 "'=''. 'ZOSSOY, being not more than one sec- 

 ond of time, or to^sooo ^f the whole amount. 



From the relative brightness of the sat- 

 ellites of Uranus, Prof. Newcomb concludes 

 that they have masses not more than soooo 

 of that of Uranus itself, i. e., vastly less 

 than the mass of our own moon. 



It is an interesting fact too that the au- 



thor suspects that the nearest of the satel- 

 lites 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 Uianns 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 in- 

 stance was any such object found to accompany 

 the planet. I think I may say, with consider- 

 able certainty, that there is no satellite within 

 2' of the planet, and outside of Oberon, having 

 one-third the brilliancy of the latter, and there- 

 fore that none of Sir William Herscbel's sup- 

 posed outer satellites can have any real exist- 

 ence. The distances of the four known satellites 

 increase in so regular a way that it cau 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 princi- 

 pal determinations of the mass of Neptune 

 have been made : 



Bond's, which gives the mass tdiso- 

 Struve's, " " " " i. 

 Lassell's, " " " " T^rss- 



From the work of the Washington tel- 

 escope the mass results tbsso, which agrees 

 most remarkably with Bond's previous de- 

 termination. 



No evidence for an elliptic form to the 

 orbits of any of these satellites has been 

 made out : " We are thus led to the remark- 

 able 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 atmos- 

 pheric conditions, during July, IS*?*." 



We have thus far spoken mainly of the 



