426 
Proceedings of the Royal Society 
Monday , 16th February 1880. 
The Eight Eey. BISHOP COTTEEILL, Vice-President, 
in the Chair. 
The following Communications were read : — 
1. On the Geology of the Eocky Mountains. By Professor 
Geikie. 
2. On Comets. By Professor Forbes. 
The author commenced by stating that although these researches 
lead him to believe in the existence of two planets revolving in orbits 
external to that of Neptune, and although there was a great deal of 
evidence to show that he had actually determined the elements of 
the orbits, yet the latter point, being dependent on a coincidence of 
probabilities only, cannot be considered a certainty until the planets 
are observed. 
The author accepts the theory of cometary orbits which supposes 
that these bodies, wandering through space, are attracted by the sun 
into the solar system so as to pursue parabolic orbits, and that some 
of these, in passing a planet, may have their velocity diminished, in 
which case they will afterwards describe an ellipse in a definite 
period. 
It has long been known that the greatest distances (aphelion 
distances) to which comets recede from the sun are grouped into 
classes. Thus there is a large class of comets whose aphelion dis- 
tance is about the same as the distance of Jupiter from the sun, and 
another large class with aphelion distance equal to Neptune’s distance. 
The author has noticed that of the other periodic comets there is a 
large group of aphelion distances 100 times as far from the sun as 
the earth is, and another about 300 times. The rest of space is very 
free from aphelion distances. This is shown by the accompanying 
table. 
At the British Association in 1879 Professor Newton of America 
proved some important propositions with respect to the introduction 
