472 Proceedings of Royal Society of Edinburgh. [sess. 
the number and increased the average size ; and the largest body, 
the moon, would capture most matter in this way. 
There is a very important difference between the collisions of 
bodies moving in the same direction of revolution and of those 
moving in opposite directions, which must he kept in view. The 
former are caused principally by bodies attracting each other ; they 
are not destructive ; and while they cause the mean distances of 
the orbits to he diminished, they tend to make these orbits less 
eccentric. The latter occur at high speeds ; they are highly 
destructive, and cause the orbits to become more eccentric. The 
moon’s moment of momentum round the earth proves that it has 
been built up principally of bodies having the same direction of 
revolution. 
The several portions which now form the moon must have long 
had independent orbits round the earth, and many may have 
grown to a considerable size before being caught by the moon. The 
moon’s mass is now an eighty-first part of that of the earth, and at 
distances of 23,800 miles (more or less, according to circumstances) 
from the moon its influence is equal to that of the earth. Hence, 
when a small body having an independent orbit round the earth 
came near the moon, it would be drawn into a subsidiary orbit with 
the moon’s centre as focus, which, with reference to the moon, 
would be a hyperbola ; and the body might strike or graze the 
moon’s surface, or escape and keep on an orbit round the earth, 
much modified by the encounter, till some other close approach, 
when it might be captured. 
With regard to bodies being captured by the earth, if two equal 
masses circulating at the same mean distance in opposite directions 
were to collide, their moments of momentum would be mutually 
destroyed, they would be highly heated and driven to pieces, and 
they would fall direct to the earth. So exact a balance as this is 
against all probability, and the most usual result of such collisions 
would be to render the resultant orbits more eccentric, and thus 
give increased chances of further collisions, because they would 
cross other orbits to a greater extent. Finally, many orbits would 
be rendered so eccentric as to cause the bodies to graze the earth’s 
atmosphere at each revolution, which would thus reduce the orbit 
till the earth captured the whole in small pieces, this effect being 
