Outer Satellite of liars in 1879. 177 



Although the opposition which occurred on November 

 12th of this year was not nearly so favourable as that of 

 1877 — the nearest approach of Mars to the earth in the for- 

 mer case being 44,866,000 miles, and in the latter only 

 35,030,000 miles — yet as elements of the orbits had been 

 computed by Professor Hall from the whole of the 1877 

 observations, which, notwithstanding that the outer satellite 

 would have made more than 600 revolutions since it was 

 last observed, he expected would not err more than 3° 

 in giving the place of the satellite in its orbit, it was 

 considered advisable to renew the search, for when once 

 the place of an object is approximately known, it is much 

 more easily seen than in sweeping for it. 



It was considered quite useless to look for Phobos, which, 

 although brighter than Deirnos, would be always so near 

 the body of the planet as to be completely overpowered by 

 its great amount of light. It may be mentioned that 

 although Mars would not approach the earth so closely as in 

 1877, yet it would be closer than during part of the time 

 while Deimos had been visible, for it had been observed at 

 Washington till Mars was- 55,273,000 miles from the earth. 

 On the night of its discovery the distance of Mars was 

 38,769,000 miles. 



Our observations were commenced on October 21st. An 

 ephemeris was computed from Hall's Elements for each night, 

 and particular attention was paid to those times when the 

 angle of position was about 53 or 233 degrees, when the 

 satellite would be at its greatest elongation from its primary, 

 the distance being then a maximum, which in the present 

 opposition would amount to 67 seconds of arc from the 

 centre, and about 49 seconds from the Hmb of the planet. To 

 provide, however, against any great error that might possibly 

 exist in the Elements Mr. Ellery instituted relays of 

 observers, so as to extend the watch throughout the night. 

 The eye-piece employed was a negative one, magnifying 255 

 times ; to protect the eye from the glare of Mars a bar of 

 blackened paper about four times the diameter of the planet 

 was placed in the focus of the eye-lens. To this was after- 

 wards added another bar at right angles to the former, thus 

 forming a cross, which greatly facilitated the estimation of 

 angles of position. During the search Mars was kept com- 

 pletely concealed behind one of these bars. 



The nights of October 21st, 22nd, and 24th were fine, but 

 nothing could be seen. The night of the 25th was unusually 



