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POPULAR SCIENCE REVIEW. 
distances are similarly related to the planet Neptune, and might he supposed 
to have been introduced into the solar system by the attraction of that 
planet. For this reason it has frequently been pointed out that there may 
be ultra-Neptunian planets with mean distances of about 45 and 75 
times that of the Earth, as there seems to be a tendency for periodical comets 
to have their aphelion distances at about one of these distances from the Sun. 
Nothing beyond mere speculation seems to have engaged the attention of 
astronomers. At the end of April, however, Prof. George Forbes published 
the following definite statement : — 1 Prof. G. Forbes, of Anderson’s University 
of Glasgow, has reason, from the study of the action of the planets on 
comets, to believe that a planet exists at present in right ascension ll h 40 m 
and North Polar distance 87°, or within, perhaps, 5° or so of that spot, 
close to the ecliptic; that the opposition is just past, and that its distance 
from the Sun is 100 times that of the Earth.’ 
Much interest was aroused by this statement, and much curiosity excited 
as to the means by which Prof. Forbes had arrived at so definite a prediction 
of the position of so distant a planet. It was obvious that he could not have 
made use of a method of inverse perturbations, like the method employed by 
Adams and Leverrier for the discovery of the position of Neptune , for, 
apart from other considerations, this would require his proving that there was 
no other planet between Neptune and this new one, which may be termed 
Forbes’ planet, and this is impracticable with our present knowledge. 
It was evident, therefore, that he must have employed some method 
based on the position and dimensions of the orbits of the known periodical 
comets. It was by no means obvious, however, how from these data there 
could be determined the present position of the planet, even granting that it 
might be legitimately assumed that such a planet had introduced certain 
comets into the solar system. 
Lately Prof. Forbes has published some details of his method of investi- 
gation. Prof. Forbes assumes, firstly, that any comet with a given aphelion 
distance has been introduced into the solar system by a planet with a mean 
distance sensibly equal to the aphelion distance of the comet ; and secondly, 
that this comet was introduced into the solar system at a time when the 
comet was at its aphelion and when the longitude of the planet was nearly 
coincident with the longitude of the aphelion of the comet. Both of these 
are fair assumptions to work on. He shows that there is a group of comets 
with period ranging between 300 and 500 years, which have their aphelion 
distance at about 100 times the mean distance of the Earth. In accordance 
with the basis of his investigation he assumes that these comets have been 
introduced into the solar system by the attraction of a planet whose mean 
distance is about 100 times that of the Earth, and whose period of revolution 
around the Sun is about 1000 years. 
Prof. Forbes then inquires what must be the position of this planet at the 
present time, so that by its motion in past ages this planet would have been 
in the right position to introduce these comets into the solar system. In a 
highly ingenious manner Prof. Forbes shows that if the present longitude of 
the planet be about 180° and it moves about £ 0 per year, it would have been 
in past ages in the right position to introduce into the solar system the 
greater number of the comets in this group. Thus, in about the year 1650 
