DOUBLE STARS 155 



this would continue until the limiting form of Maclaurin's 

 spheroid was reached. Beyond this, upon further con- 

 traction, the form of Jacobi's ellipsoid would be assumed 

 and the period of rotation would increase. The ellipsoid 

 would become longer in proportion to its girth, and on 

 passing the critical condition of fig. 7, would be replaced 

 by the pear. In the development of the pear mathemat- 

 ical methods fail, but there can be little doubt that its 

 form becomes more pronounced, and that a waist develops 

 that ultimately divides the mass into two bodies, nearly in 

 contact, and in rapid revolution. At this point exact 

 methods resume their sway. Each of the separate masses 

 produces tides in the other, their axial revolution is 

 checked, and the distance between them increases. From 

 this point their history is that of the Earth and Moon. 



The reader will be inclined to ask whether this scheme 

 may not replace that of Laplace, and whether the planets 

 may not have been developed from the Sun, and sub- 

 sequently thrown off to their present distances by 

 tidal action, as the Moon appears to have been deve- 

 loped and thrown outward from the Earth. There are, 

 however, fatal objections to this rather attractive view. 

 By imagining their tidal effects undone, the Moon and the 

 Earth have been traced back to a condition when they 

 must have been very nearly in contact. A similar oper- 

 ation performed upon the Sun and Earth reduces the 

 Earth's distance from the Sun by quite an insignificant 

 amount. The Earth must have been formed not far from 

 the present position of its orbit A similar objection 

 appears in any attempt to explain the origin of the satel- 

 lites of any other planet in this way. The system of the 

 Earth and Moon is unique in the solar system. In no 

 other case does a satellite possess nearly so great a mass 

 in proportion to that of its primary, and it is owing to 



