ASTRONOMICAL PHENOMENA AND PROGRESS. 



ably broader than the streak, and, what is very 

 remarkable, portions of blue streak appeared 

 on the north and south sides of the square 

 patch, as though the light patch had been 

 formed from material drawn away from the 

 general covering of the surface, thus leaving 

 vacant spaces both above and below it." 



In "The Observatory" for April, 1883, W. 

 F. Denning gives some results of his own ob- 

 servations of Jupiter's equatorial white spot, 

 and also of the great red spot now no longer 

 visible. Mr. Denning has found that while 

 the white spot was completing 2,064 revolu- 

 tions, the red spot performed only 2,045 ; in 

 other words, the white spot gained 19 revolu- 

 tions. The former, therefore, moves 260 miles 

 an hour more rapidly in a direction from west 

 to east around the planet. The average pe- 

 riod of the red spot from July, 1881, to March, 

 1883, was 9 h - 55 m> 37'7 8 - ; that of the white 

 spot, 9 h - 50 ra- 8/7" - . These results are very 

 nearly identical with those found by Prof. 

 Hough. 



Researches on the Satnrnlan System. During the 

 past three years Dr. Wilhelm Meyer, of Gene- 

 va, has been engaged in an elaborate investi- 

 gation of the Saturnian system.* His observa- 

 tions of 1881 give the following: 



Dimensions of Saturn and its rings for the distance. . 

 Exterior diameter of the bright ring. ................ 



Diameter of the ring in the middle of Cassini's di- 



vision .......................................... 



Interior diameter of the bright ring ............ _____ 



Interior diameter of the dusky ring ................ 



Distance between the extremity of the ring and the 



planet on the west ........ ....................... 



The same distance on the east ...................... 



Equatorial diameter of the planet ................... 



Polar diameter of the planet ........................ 



Compression ...................................... ^^ 



The interior edge of the bright ring was not 

 sharply defined, and hence the interior diame- 

 ter could not be determined with accuracy. 



Orbits of the Satellites and Mass of the Primary. 



Dr. Meyer's observations of Mimas and Hype- 

 rion were insufficient for a determination of 

 their orbits. The others were satisfactorily 

 observed, and the resulting distances and pe- 

 riods are given below : 



SATELLITES. Mean distance. Period. 



d. h. in. e. 



Enceladus ....................... 34"'3501 1 8 53 6'92 



Tethys ......................... 4'2 -7514 121 1825-62 



Dione ........ ................... 54 -7574 217 41 9'29 



Ehea ........................... 76 -4838 4 12 25 11'57 



Titan ............................ 176 '9102 15 22 41 23'16 



lapetus .......................... 514-710879 7 4924-84 



The mass of Saturn obtained from these peri- 

 ods is ^4^, a value somewhat greater than 

 that found by Bessel. The mass of the ring, 

 that of Saturn being 1, is y^. 



The Divisions in the Ring. The London " Ob- 

 servatory" for September, 1883, gives the fol- 

 lowing abstract of Dr. Meyer's researches on 

 the divisions of Saturn's ring, and the disturb- 

 ing influence of the satellites: 



" Prof. Kirkwood showed, some twenty years 

 ago, that Jupiter exercised a peculiar influence 



*Astr. Nach., Nos. 2,517,2.527; London Obs., July and 

 September, 1883; Payne's Sid. Mess., September, 1888. 



84-48 

 26 '05 

 21 13 



11-34 

 11 -30 

 17' 77 

 16-12 



over the minor planets, tending to produce- 

 well-marked gaps among them at certain well- 

 defined distances. For if the period of any 

 minor planet were commensurable with that 

 of Jupiter, the latter would exercise a perturb- 

 ing influence upon it, which would eventually 

 result in a complete change of orbit. Later 

 on, in 1868, Prof. Kirkwood employed the 

 same principle to account for the great divis- 

 ion (Cassini's) in Saturn's rings. Maxwell had 

 shown that the rings must be formed of sepa- 

 rate particles moving round the planet to a 

 certain extent as independent satellites. But 

 a body moving round Saturn at the distance of 

 Cassini's division would have a period that was 

 very closely commensurable with those of each 

 of the six inner satellites, and it would, there- 

 fore, be especially exposed to perturbation. 

 Dr. Meyer has carried the principle yet further, 

 and has investigated every possible combina- 

 tion of the commensurabilities of the revolu- 

 tion periods of the satellites, and he finds that, 

 including the division of Cassini, there are 

 seven places where the satellites would unite 

 to exercise a perturbing influence on the mem- 

 bers of the ring system. The first position is 

 where the period would be one fourth of that 

 of Mimas, and marks the inner boundary of 

 the dark ring. Particles moving at almost 

 precisely the same distances would have their 

 times commensurable with each of the other 

 five inner satellites: thus, for a period of one 

 fourth of that of Mimas, we have a distance of 

 10'56"from the center of Saturn ; for one sixth 

 of that of Enceladus, 10-43", and for one eighth 

 of that of Tethys, 1Q-66". Dr. Meyer sees a 

 consequence of this close agreement in the 

 well-defined character of the inner edge of the 

 dark ring. Next comes Struve's division in 

 the dark ring. One fifth the period of Ence- 

 ladus corresponds to a distance of 11-79", one 

 seventh that of Tethys, 11 -66"; the next three 

 satellites give a closely similar result. The 

 position of Struve's division is not very exactly 

 known, and Dr. Meyer adopts 11*79" as its 

 distance, being the mean between the posi- 

 tions of the inner boundaries of rings C and 

 B. One third of the period of Mimas intro- 

 duces a new series of commensurabilities in 

 which all the six satellites take part, but the 

 agreement is by no means so close as in the 

 first two cases ; and Dr. Meyer regards the in- 

 distinct character of the inner boundary of the 

 bright ring B, which would about correspond 

 to the mean of the distances indicated, as con- 

 nected with this less perfect coincidence. The 

 period of Enceladus is four times, that of Tethys 

 six times, that belonging to a particle at this 

 distance. Cassini's division corresponds, as'al- 

 ready stated, to a period commensurable with 

 each of the six inner satellites, the period of 

 Mimas being twice as long, Enceladus three 

 times, Tethys four, Dione six, Rhea nine, Ti- 

 tan thirty-three. The commensurabilities in 

 the case of the four nearest satellites are of 

 the simplest possible character; and we find 



