ASTRONOMY. 31 



these satellites. "Expressing the mass of the planet in the 

 common unit, we have, from the above values of the elements, 

 the following results : 



v 1 



Deimos: Mass of Mars 



Phobos: Mass of Mars =^ 



3095313 3485 

 1 



3078456 10104' 



" These results agree so nearly within the limits of their 

 probable errors that I have taken the mean by weights as 

 the final result from the Washington observations. In this 

 way w T e have 



Mass of Mars=- ; : ." 



3003500 3205 



The recent report of Professor Pickering, Director of Har- 

 vard College Observatory, states that it has been decided to 

 devote the large refractor chiefly to photometry. In this 

 way a field is taken up which has too long been unoccupied. 

 Besides a great number of photometric observations on dou- 

 ble stars, asteroids, and satellites of the outer planets, the 

 satellites of Mars have been studied. Assuming the albedo, 

 or intrinsic reflecting power of these bodies, to be the same 

 as that of Mars, it is concluded that the diameters are for 

 Deimos (outer satellite) about 6 miles, and for Phobos (inner) 

 about 6.5 miles. 



Following are some previous values of the mass of Mars 

 which are interesting for comparison with Professor Hall's 

 results : Laplace assumed the mass of Mars to be la4 S 08 -g. 

 Delambre reduced this estimate to tj 54 * ^ -^j. Burckhardt, 

 in 1816, diminished this still further to -^mruTST- By Han- 

 sen and Olufsen, of Sweden, in their solar tables, the esti- 

 mate is S2UUUUU- Leverrier got ^yrnrnro- 



The spectrum of Mars has been photographed by Dr. 

 Huggins. 



Professor Hall, of Washington, made, daring the last opposi- 

 tion of Mars, a, large number of measures (thirty-two nights) 

 of the position of the south polar spot of Mars. From all 

 of these he finds for the angle of position of this spot for 

 1877, September 17.0 G. M. T., 166 22', and for the same 

 epoch the radius of the small circle described by the spot is 

 5 11'. The various determinations of the south polar dis- 



