Page:The Solar System - Six Lectures - Lowell.djvu/142



Moreover, it is directed in both cases along the same line. Whence its moment in the two cases about any point is the same in amount, the perpendicular from the point being common to both, but opposite in direction. The two moments thus destroy one another. From which we see that the internal forces of a system are unable to change the moment of momentum of the system. Similarly they are incapable of having created it to begin with.

The present moment of momentum of the solar system can be calculated. It is found to be nearly the least possible. It must, therefore, always have been so. It was predestined by internal motions to make a single star.

So far, he is admirable, but from this point I lose him; I cannot see the cogency of all his succeeding steps. They lead him to the conclusion that everything is as it should be, and incidentally that Jupiter or Neptune is the oldest planet, Uranus the next, then Saturn, Mars, the Earth, Venus, and Mercury. The importance of the order will appear shortly.

With regard to the retrograde rotations of the outer planets and the direct rotations of the inner ones, Trowbridge suggested that uniform density, or a density increasing toward the centre, would