Page:The Solar System - Six Lectures - Lowell.djvu/137



tell you. For it was founded on what it has now foundered on: one fundamental mistake. Laplace assumed that his nebula would revolve, as he saw the air around the Earth to revolve, of a piece. But he forgot that friction due to the pressure alone produces this, and that in particles moving freely no pressure exists. Under the pull of a central mass each layer of the nebula would revolve at its own appropriate rate, or as $$\scriptstyle r^{-\frac{1}{2}}$$. So that his beautiful explanation of the agreement in direction of the rotations and the revolutions—the vital point of the theory falls to the ground.

Faye first definitely pointed out this fatal fallacy in Laplace's hypothesis in 1886, in his "Origine du Monde," in which, after reviewing the previous history of the subject, he brought forward a new theory of his own, both elegant and ingenious.

He begins by assuming a nebulous mass of particles, roughly uniform throughout, but with local condensations. He supposes this nebula cold, for the heat can be trusted to come of itself. With uniform density throughout, the speed of rotation would also be uniform, thus giving the same result that Laplace got, but for a very different reason. In a spherical mass of matter of uniform density, a particle at any point is attracted only