Page:The Solar System - Six Lectures - Lowell.djvu/139



Planets given off under the first state of things would rotate in the same direction in which they revolved; under the last in the opposite way. He, therefore, supposes the terrestrial planets to be the older; the outer planets the younger members of the system. His theory makes the order of birth the exact contrary of Laplace's.

More recently Lieutenant-Colonel R. du Ligondés has evolved another cosmogony. Ligondés's general theory is ingenious, but to me not convincing. His first point is unqualifiedly good. He starts out by calling attention to the evidence offered by the moment of momentum of the solar system upon the early history of that system. He shows that to produce a single star system like ours, the original motions of the several parts of the nebula must have been nearly balanced, the plus motions almost canceling the minus ones.

It now becomes of interest for us to consider this question. Conservation of moment of momentum is as fundamental in mechanics as the conservation of energy. The momentum of a body is its mass into its velocity, and the moment of momentum is this mass-velocity multiplied by the perpendicular upon its direction from the point