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POPULAR SCIENCE REVIEW. 
peculiar red spot is slowly moving around Jupiter — an interesting point 
which ought to be carefully investigated. 
Another feature, which has been very striking during the last opposition 
of Jupiter, is the common occurrence of round or oval, bright white spots. 
So bright and circular have these objects been that they could easily be 
taken for satellites in transit. These spots have been long known to occur. 
Some time back Mr. Brett pointed out that they were bordered on one side 
by shades, as if they actually cast shadows. Of late, however, it has been 
noticed that these shadings occur on both sides of tbe spots, and even all 
round, throwing them up, as it were, into relief. Under these circumstances, 
it is evident that these dusky borders cannot be shadows cast by the white 
spots, as supposed by Mr. Brett. Mr. Corder has concluded from his observa- 
tions that these white spots are moving much quicker than the great 
red spot, as he is of opinion that he has been able to see one of these spots 
gain half a revolution of the planet on the red spot during a period of only 
twenty days. 
Satellites of Mars. — The outer satellite of Mars, Deimos, was observed by 
Mr. A. Common, of Ealing, with his fine 37-inch reflector, on September 
21st, 1879, more than three weeks before it was anticipated it could be seen 
with even the great Washington Equatorial. Later Mr. Common succeeded 
in observing the inner satellite, Phobos ; and since then the two satellites 
of Mars have been regularly observed and measured at the Ealing Observa- 
tory. At Washington the outer satellite, Deimos, was seen on October 10th, 
the very day it was anticipated that it would come within reach of the great 
26-inch equatorial. Since October 12 both satellites have been regularly 
observed by Professor Asaph Hall, at Washington. A measure of the 
position of Deimos was also obtained by Mr. Maunder, with the 12f-inch 
equatorial of the Royal Observatory at Greenwich. Several observa- 
tions have also been obtained at the Paris Observatory. At Dun Echt 
Observatory, Aberdeen, Lord Lindsay saw the first satellite with the 
lo-inch equatorial, but clouds prevented its being measured. Mr. Common 
is of opinion that any one who can see Enceladus, the second satellite of 
Saturn, ought to be able to see with ease both satellites of Mars, if only the 
precaution be taken to hide the planet behind a bar. He is of opinion 
that the outer satellite, Deimos, is quite as bright as Enceladus. Phobos, 
though harder to see from its proximity to the planet, is known to be far 
blighter than Deimos, probably as bright as Tethys. 
By comparing the observations of the two satellites with the ephemeris 
founded on the observations made during 1877, Professor Asaph Hall finds 
that Phobos comes to its elongation 44 m, 0 before the predicted times, the 
error in areocentric longitude of his ephemeris being + 34 0, 3, so that the 
periodic time ought to be reduced by 1 8 *074, and would be 7 h 39 m 13 s *966. 
In the case of Deimos , the outer satellite, the error of the ephemeris is very 
small. This correction will bring the value of the mass of Mars derived from 
the inner satellite into better agreement with the mass of Mars as deter- 
mined from the motion of the outer satellite. 
