If the nebula were not in rapid rotation, a slight disturbing cause, acting at the critical moment when a planet was being ejected, might determine the inclination of the planet’s orbit, which might thus be very oblique to the equatorial plane of the nebula. Thus the hypothesis is not open to one of the objections which have been urged against that of Laplace--namely, that the orbits of some of the planets in the solar system are inclined at a large angle with the plane of the sun’s equator. [Illustration: This illustrates Laplace’s theory, which conceived of a vast nebula filling the whole space of the solar system and rotating around a central axis. The outer and thinner part had much greater movement than the denser central mass, finally being thrown off as a ring, which in turn rolled up into a ball, still following the same course as the ring had followed. Thus the earth broke off from the sun and the moon from the earth. The theory is, however, no longer credited by scientists. The pear-shaped nebula is the theory of a young English mathematician, Professor J. H. Jeans. Starting with a spherical nebula, he argues that in cooling it will assume the form illustrated above, and that the smaller part will separate and form a satellite rotating independently but within a distance influenced by the parent mass. The spiral nebula in Canes Venatici, a revolving mass of gas or meteorites, supplies, according to the nebular hypothesis of Messrs. Chamberlin and Moulton, an excellent example of how the earth and moon were formed. We may reasonably imagine the smaller spiral to represent the moon in the act of being thrown off by the earth. THREE FAMOUS THEORIES OF THE BEGINNING OF THE EARTH] [Sidenote: Heavenly Bodies in Collision] Jeans mentions two disturbing causes in particular which might easily arise--one the penetration of the nebula by a wandering meteorite, which might precipitate an event already on the verge of happening, and simultaneously determine both the birth of a planet and the obliquity of its orbit; the second, the presence of some distant mass, such as a star, which, by raising a quasi-tide in the nebula, would give the final touch required to overturn its equilibrium. The influence of a distant body, such as a passing star, has been invoked by Moulton in another version of the nebular hypothesis. In conjunction with Chamberlin, he calls special attention to the spiral nebulæ, which are by far the commonest kind, as presenting the closest approach to the conditions which obtain when planets are actually in course of formation. Chamberlin and Moulton enter on a detailed account of the manner in which they suppose the planets to have grown by the gradual accretion of meteoric masses as these encountered each other while moving in various elliptical orbits.