SCIENTIFIC SUMMARY. 
271 
three appearances cannot be brought into agreement unless it be assumed, (1) 
that the mass of the planet Jupiter must be reduced from Bessel’s value 
1 -f- 1048 to 1 4- 1051 ; or (2) that the comet suffers a similar retardation in 
its motion to that which Prof. Encke pointed out was indicated by the motion 
of Encke’s comet. The former of these two alternatives seems out of the 
question, as all recent investigation of the mass of Jupiter from the motion of 
its satellites, as well as Prof. Axel Holler’s researches on the motion of Faye’s 
Comet, and Prof. Kruger’s investigation of the perturbation of the minor 
planet Themis, agree in confirming Bessel’s value for the mass of Jupiter. 
It would appear, therefore, that Winnecke’s Comet really experiences a similar 
retardation of its motion to that which seems to exist in the case of Encke’s 
Comet ; this retardation, as it is well known, by diminishing the dimensions of 
the orbit shortens its period of revolution. Accordingly Prof, von Oppolzer 
finds, after each appearance, a mean daily acceleration in its sidereal motion 
of 0"*01439. This, as Prof. Oppolzer shows, corresponds to a value of the 
retardation in close agreement with that found by Encke from the motion of 
the famous comet bearing his name. Of late years the existence of this 
resisting medium has been considered doubtful, for Encke’s Comet seems to 
be the only one showing any well-defined trace of its effects. Prof. Axel 
Holler had most carefully investigated the motion of Faye’s Comet, another 
of short period, but failed to find any trace of such retardation. Prof. 
Oppolzer is of opinion that this is not surprising, as the effect of such 
a resisting medium on Faye’s Comet would be so small that its effects could 
not be disentangled from the effects of the unknown errors in the per- 
turbations. 
The question is one of very great interest, and it is very desirable that 
as much light as possible should be thrown on the subject. It requires a 
rigid and exhaustive investigation of the motion of all the comets of short 
period — a work of very great labour. It is probably only in this manner 
that further light can be thrown on the subject. 
The Great Southern Comet, 1880. — From the further particulars which 
have now reached England, this comet seems to have been a bright comet of 
considerable size, and if visible, under more favourable conditions, would 
have been probably of great brilliancy. It moved in an orbit which approached 
very close to the Sun, so that the comet swept round its perihelion passage 
with enormous velocity, and rapidly rushed off into space. It was not 
seen until several days after perihelion passage, when its tail was observed 
as a bright band rising above the western horizon. The comet was moving 
tail foremost, and the nucleus could not be seen ; and even the tail, which was 
of great length, was only visible in bright twilight. The comet grew rapidly 
fainter, and as it receded from the Sun the nucleus could be detected as a 
small bright mass. Long before the end of the month, however, the comet 
had become so faint that it could no longer be observed, even with the great 
Helbourne reflector. It is probable, that under more favourable circum- 
stances, this comet would have taken rank as one of the largest and most 
brilhant comets of the century. It never rose sufficiently above the horizon 
for even its tail to be visible in England. 
Immediately on the arrival of the observations in Europe, the calculation 
of the orbit of the comet was undertaken by astronomers, and especially by 
