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POPULAR SCIENCE REVIEW. 
the mean density of the planet to he less than that of any other. Its appa- 
rent diameter would he over 6", so that it would appear like an exceedingly 
bright planetary nebula, and could not well he mistaken for a star by the 
Greenwich observers. Moreover, it would certainly produce well-marked 
perturbations in the motion of the greater planets. This consideration seems to 
render it out of the question that the missing star from the Greenwich 
Catalogue can be Forbes’ planet. For it to he really rendered possible for 
this star to he Forbes’ planet, the planet must he supposed to shine very 
brightly by its own light, and to he of nearly the dimensions of the planet 
Jupiter. It would then shine like a ninth-magnitude star, and might he 
detected without very great difficulty. The assumption, however, that a 
planet of this size could he shining brightly by its own light, is one which 
is far from being probable. Even under the most favourable circumstances 
it is not likely that an Ultra-Neptunian planet would possess greater dimen- 
sions than the planet Jupiter. This would render its apparent diameter 
about equal to that of Neptune , and its brightness to that of a star of 
the eleventh magnitude. The planet’s motion being so slow, it would not he 
an easy object to detect, but it is not improbable that a long search with a 
good instrument over an area of 5° by 2° would enable such a body to be 
detected if it existed. 
From the above consideration it would appear, therefore, that even 
if Prof. Forbes’ planet really does exist, its detection cannot he an easy 
matter. 
On a Suspected Variation in the Position of the Earth's Axis . — It has 
been pointed out, that if from any cause the axis of rotation of the Earth does 
not coincide with the axis of figure, the former will revolve round the latter 
in a period of about 306 days. Accordingly there' ought to appear a small 
variation in the apparent latitude of Greenwich, deduced from observation 
of the Pole-star, which would go through all its variations in about ten 
months. Professor Clerk Maxwell endeavoured to ascertain whether the 
Greenwich Observations for the years 1851-54 gave any indication of the 
existence of such a variation, but with doubtful results. The subject has 
been lately taken up by Mr. A. Downing, of the Royal Observatory, who 
has discussed the observations of Polaris made between 1868 and 1877. He 
finds evidence of a small periodical variation with a ten months’ period. 
The co-efficient of this term amounts, however, to only 0"'075 ± 0"T5. 
This would indicate that the axis of figure of the Earth revolved round 
the axis of rotation in a circle, about six feet in radius, once in ten months. 
This result is in close accordance with one derived by Professor Peters 
from a discussion of the observations made at Pulkowa. On the other hand, 
Dr. Nyr6n has arrived at values for this variation in latitude at these different 
periods, which are discordant with each other, and would indicate some 
different source as that of the variation. 
Winnecke's Comet . — In the Astronomischen Nachrichten, No. 2014, there is 
an important note by Prof, von Oppolzer giving some of the results of his recent 
researches on the motion of Winnecke’s Comet. This is a well-known comet, of 
short period, and has been carefully observed during its appearances in 1858, 
1867, and 1875. From his investigation of the perturbations in the motion of 
the comet, Prof, von Oppolzer shows that the observations made at these 
