ASTK'OXOMY FOR 1889, 1890. 145 



coordinates of these two j>laiiets, siviiiQ also a provisional comparison of 

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

 of Hansen. 



In conimenting upon recent determinations of planetary masses from 

 the motions of comets, Professor Hall says : 



"The objection to deducing valuer of planetary masses from the 

 motions of comets consists, I thinii, in the fact that apparently other 

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

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

 ott" to form a tail. Until w^e know more of the theory of these changes 

 the computation 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 confirm his 

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

 regarded as one of the unsettled problems of astronomy. M. Schiapa- ' 

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

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

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

 of 88 days, the same, in fact, as the period of rotation of the planet / 

 around the sun. Schiaparelli also concludes that the axis of rotation , 

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

 being uniform. 



Dr. von Haerdtl has obtained the following values for the mass of 

 Mercury : 



I. Mass of Mercury, 1: 5,012,842 from Win uecke's comet. 



II. Mass of Mercury, 1: 5,514,700 Le Verrier's eqnatiou modified. 



III. Mass of Mercury, 1: 5,64H,(i00 Encke's comet, 1819-18Ud. 



IV. Mass of Mercury, 1: 5,669,700 Encke's comet, li:;71-1885. 



Venus. — Schiai)arelli has concluded, from an exhaustive rediscussion 

 of 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 allusion has been made in previous reports, 

 continues to receive considerable attention from astronomers and 

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

 ner, in continuation of his former researches, from 7 pairs of scars at 

 three different times of the year: 



Epoch. Latitude of Berlin. 



1884. 32 +.52° 30' 16". 73-0. 82 ^ A 



1884. 70 16".y6-f 0.83 A A 



188.3. 31 16"..^)2— 0.85 A A 



H. Mis. 129 10 



