ABTliOyOMY FOR 1889, 1890. 145 



ooordiuates of tbt^se two pliiiiets, ^iviiis also a provisional coiupaiisou of 

 his theory with observations. The method followed is in general that 

 of Hansen. 



In coinnu'iiting upon recent determinations of planetary masses from 

 the motions of comets, Professor Hall says: 



"The objection to dednciiii!,' values, of planetary masses from the 

 motions of comets consists, 1 thin if, in tiie fact that apparently other 

 forces than tluit of gravitation act on these bodies. As a comet ap- 

 proaches the sun it changes form, disintegrates, and matter is thrown 

 off to form a tail. Until we know more of the theory of these changes 

 the com[)utati()n of masses from the motions of comets and inferences 

 about the resisting medium in space must be uncertain." 



Mercury. — The observations of Schroter early in the present cen- 

 tury indicated that Mercury had a motion of rotation about its axis of 

 about 24 hours. Subsequent observers failed, however, to contirui his 

 observations, and the question of Mercury's rotation has generally been 

 regarded as one of the unsettled i)roblems of astronomy. M. Schiapa- 

 relli, taking advantage of the clear sky of Mihsn, has observed Mercury 

 since 1881, obtaining about one hundred and tifty sketches, showing 

 quite well marked spots, from which he has deduced a rotation period 

 of SS days, the same, in fact, as the period of rotation of the planet 

 around the sun. Scliiaparelli also concludes that the axis of rotation 

 must be nearly [perpendicular to the orbit of the planet, the rotation 

 being uniform. 



Dr. von li;erdtl has obtained the following values for the mass of 

 Mercury : 



I. M;iss of Mercury, 1: .3,012,842 from Winiiecke's comet. 



II. Mass of Mercury, 1: 5,.514,700 Le Verrier's eiiuation moditied. 



III. M:is8 of Mercury, 1: 5,(i4H,(i00 Eucko's comet, 1819-18(58. 



IV. Mass of Mercury, 1: 5,009,700 Encke's comet, If^Tl-lrtSr). 



Venxts. — Schiapaielli has concluded, from an exhaustive rediscussion 

 ol all the older observations, combined with his own observations of 

 1877 and 1878, that Venus rotates upon its axis in 225 days, or the same 

 time that it rotates about the sun, contrary to the generally received 

 hypothesis that its rotation period is about 23 hours. Venus, then, as 

 well as Mercury, would seem to turn always the same face to the sun, 

 as the moon turns the same face to the earth. 



The Earth — Variation of latitude. — The subject of the change of ter- 

 restrial latitudes, to which allusiou has been made in previous reports, 

 continues to receive considerable attention IVom astronomers and 

 geographers. The following results have been obtained by Dr. Kiist- 

 ner, in continuation of his former researches, from 7 i)airs of stars at 

 three different times of the year : 



Epoch. Latitude of Berlin. 



1884. 32 -}-.')2o :50' 16".7:5— 0.82 J A 



1884.70 10".96-f 0.83 J A 



lH8r.. 31. l(;"..-.2— 0.ft5 z/ A 



H. Mis. 129 10 



