1900-1901.] Prof. Forbes on the Ultra- Neptunian Planet. 371 
bahnen, 1893) which could throw light on the subject. The 
results are interesting, and generally confirm the conclusions 
arrived at in 1880 as to the probable position of the new planet. 
The whole of the work was gone over, reasons were found for 
altering some of the data, an error in one of the calculations was 
discovered, and a comparatively recent comet was added to the 
list. Yet the final position assigned to the planet was unchanged. 
The present c Note ’ comes from the discovery of a remarkable 
confirmation of these results. It is well known that the comet of 
1556, which has generally been looked on as a return of the 
comet of 1264, did not reappear in 1848 as was expected. In 
fact, it seems to have disappeared as completely as did Lexell’s 
comet of 1770 by the attraction of Jupiter upon it when in 
aphelion. 
The longitude of the aphelion of comet 1556 was 99 0, 24' in the 
year 1696, and its distance from the sun was 88 times that of the 
earth. Now, I find that if my published results be correct the 
longitude of the new planet in 1696 was 112°, its distance from 
the sun being 100 times that of the earth. From this it appears 
to be highly probable that the non-return of the comet was due to 
its deflection at aphelion by the new planet. 
Anyone who has read Laplace’s analysis of the action of Jupiter 
upon Lexell’s comet * must realise that if Jupiter’s longitude had 
been unknown it might have been found by the action upon the 
comet. So also in this case we may deduce conclusions which 
must be true if the comets 1264 and 1553 were identical. And 
the first conclusion is that the longitude I have assigned to the 
planet which we know to be at 100 times the earth’s distance from 
the sun is not far wrong. 
The latitude of the comet 1556, when in aphelion, was 30°. 
Hence its distance from the planet was very much greater than 
is the case with ordinary cometary perturbations considered by 
astronomers. On the other hand, such perturbations are im- 
portant only for a few days or weeks, while in the present case 
the influence remains of the same order of magnitude for nearly 
two hundred years. 
It becomes then a matter of great interest to examine, generally 
* Mecanique Celeste , vol. iv. , pp. xviii. and 223, etc. 
