Outer Satellite of Mars in 1879. 179 



same object with comparative ease this evening, as likewise 

 did Mr. Grieve, the assistant workman ; but no measures 

 could be taken, for the body always disappeared when it was 

 brought near the bars. 



On November 17th the best position was near the time 

 of sunset. The night was fine, but Mars was too low, and 

 for part of the time obscured by Government House. Just 

 before 9h. Om., when it became cloudy, Mr. Turner suspected 

 a faint speck of light in the assigned position. 



The night of November 20th was very cloudy till lOh. 0m., 

 when it became very fine; the presence of the moon, 

 however, interfered with the observations. A few fitful 

 glimpses of the satellite were seen by Messrs. Ellery, 

 Turner, Grieve, and myself, but the observation was so 

 difficult that nothing like the same certainty was felt as 

 on the former occasions. 



At the time, of our last observation Mars was 46,977,000 

 miles from the earth ; at his next opposition in December, 

 1881, his nearest approach will be 56,063,000 miles. This 

 will gradually increase till the opposition of 1886, after 

 which a diminution will take place ; so that, unless we have 

 improved optical means, I think there is little chance of 

 seeing those very minute bodies before 1890 or 1892. 



The only other observations of Deimos at this opposition 

 which have come under my notice are those of Mr. Common, 

 in the south of England. He obtained his first observa- 

 tion on September 21st, or about three weeks before it was 

 expected to be seen with the great Washington refractor, 

 with which the original discovery was made. Mr. Common's 

 instrument is a silver on glass reflector, of three feet in 

 diameter. Mr. Common estimates the error of place in the 

 orbit to be about 15°, corresponding to 1*3 hours. The 

 estimates of position given by the Melbourne observers 

 are necessarily very rough, for it was absolutely necessary 

 to keep Mars completely behind the occulting bars to see 

 the satellite at all. 



