Dr. Wentwortse Erck on the Satellites of Mars. O73) 
have sufficed to have shown the satellite if it could have been removed away from 
the overpowering splendour of the planet itself; so that, notwithstanding all our 
precautions to exclude the light of the planet, fully nine-tenths of the light of the 
satellite were overwhelmed thereby, and without the precautions adopted the 
satellite would have been utterly invisible. 
My observations of the satellite were as follows, and at the time of making them 
I had no knowledge whatever of its place; indeed, the first Ephemeris published 
was that in Nature, of the 13th September, previous to seeing which I had 
communicated my observations of the 4th, 8th, and 15th. 
The times are Greenwich sidereal :— 
— | Epoch, 1877. Position. Observations. 
September, ‘ ( 3 23) 0 64° | Distance from limb, perhaps 3 diameters. 
5 |< 4 OO - Distance now estimated at 2 diameters. 
e A i) 4 Ihe) nde 
Fr 5 8 22 35 78 | Cloudy. 
% . ( a) Hil, UO 257 | 
is : ao 23 30 143 | Watched the motion during interval. 
> : i 16 1 oo 268 
% 2 | (25 AL 20 270 | Gibbosity conspicuous; Satellite very much fainter than 
; formerly. 
“3 : 4 v4 DY XN) 264° | Sufficient moonlight to read Nautical Almanack. 
; rec” Ore Sonal) 255: 
¥3 hy 27 3), (0) 69° | Mean of 2 measures ; distance perhaps 3 diameters. It is 
| exceedingly faint ; I can scarcely see it. 
October, . sale vo 24. - 70° | I think, but only think, I see the Satellite about this posi- 
tion ; however, on referring to the Ephemeris, I find 
that such was its place at that time. 
This was the last time I saw the satellite, though I looked for it under favourable 
circumstances on the 4th, 8th, and 11th October. 
At transit on the 27th September, when the satellite had become all but invisible 
to me, the log. distance of the planet was 9.625; and the log. radius vector, was 
0.143, so that the brilliancy of the planet at this time was but 0.82 of his brilhancy 
on 5th September, the day of opposition, 
Of all the oppositions since 1849 the only one at which the brilliancy exceeded 
0.82 was that of 1860, when the brilliancy on 20th July was 0.93; but the twilight 
at that season of the year would probably have extinguished the satellite. Similar 
considerations will probably suffice to show how easily the satellite may have escaped 
detection in former years. 
It does not appear probable that the satellite varies very much in brightness, 
for it was as well seen in the preceding as in the following half of its orbit ; in the 
former from 238° to 270°; in the latter from 55° to 78°. 
As to the size of this satellite, seeing that it does not present any sensible 
disc (indeed, the actual disc cannot be a tenth of a second), it only remains 
