SCIENTIFIC SUMMARY. 
269 
it might have introduced the comet, I. of 1843, and in the year 1608 have 
just moved into the right position to introduce comet II. of 1850. Still 
earlier, in 968, it would have been in the position to introduce comet IV. of 
1840, and in the year 409 the comet I. of 1861. Assuming, then, the planet 
did introduce these comets at these times, Prof. Forbes is able to show that 
the planet must have a period of about 1006 years, a mean distance of 100|, 
and be in longitude 174° in the year 1880. Consequently its present position 
would be Right Ascension ll h 40 m , and North Polar Distance 87°. Prof. 
Forbes then inquires if there be any stars now missing from the Catalogues 
which may be really observations of this planet. The only one he finds is 
star No. 894 of the Greemvich First Seven-Year Catalogue. This is a star which 
was observed twice in the year 1857, but on no subsequent occasion, and its mag- 
nitude is not stated, but cannot have been less than eighth or ninth magni- 
tude. It is not unlikely that this supposed star may have had no existence, 
and the observation have been that of some known star, written down with a 
wrong reading, or it may have been a faint star observed by mistake, having 
been taken for a minor planet, which would account for its not being 
observed again ; but, as Prof. Forbes points out, it may have been his 
planet, which would have been quite close to the place at the time. 
If this planet really exists, what would be its probable dimensions and 
appearance? Uranus and Neptune are both about 35,000 miles in dia- 
meter, and it may be assumed that the new planet is of the same dimen- 
sion. Then, from its great distance, it would present a disc only 0"'8 in 
diameter, or practically undistinguishable in size from that of a star, unless 
it be with a very large and perfect instrument. Its brightness would also 
be much less than that of Neptune, as not only would it receive but one tenth 
of the amount of light, but there would be only one-tenth as much reflected to 
the Earth, so that it would shine with only one-hundredth of the brightness. 
The brightness of Neptune is that of a star of the eighth magnitude, so that 
the brightness of the new planet would be only that of a star of the fourteenth 
magnitude. Its mean daily heliocentric motion would be 3^" of arc, which 
would be so slow that it could only be detected after several days’ interval. 
Considering, therefore, the very great number of fourteenth-magnitude stars 
which there are in any small area in this portion of the heavens, probably many 
hundreds in the area which must be searched to find this planet, and the 
necessity that there would be of mapping the whole district as practically the 
only way of detecting so slow a motion, it may be regarded as almost hope- 
less to search for such a body. It is probable that the planet would not even 
be visible with less aperture than 9 or 10 inches, and it would require a 
telescope of at least 15 or 16 inches aperture to properly search for it. 
Unless, therefore, this new planet of Forbes is much larger than either 
Neptune or Uranus, its discovery is well-nigh hopeless. 
But if the missing star from the Greenwich First Seven-Year Catalogue 
be really this new planet, it must shine like a star of the eighth or ninth 
magnitude. To do this it must have a diameter at least eight times that of 
Neptune or Uranus. This would be excessively improbable, for it would 
imply a diameter of 280,000 miles, or three times that of Jupiter and 
nearly one third that of the Sun, whilst its mass would probably be nearly 
twenty times that of Jupiter or one-fiftieth of that of the Sun, assuming 
