744 Dr. H. Wilde on the Origin of 



same distance from the sun as Ceres, and had been shattered 

 by some internal convulsion. This hypothesis was put for- 

 ward by Gibers the discoverer of Pallas in 1802, and was 

 made the subject of a memoir by Lagrange, in which he 

 determined the explosive force necessary to detach a fragment 

 of a planet that would cause it to describe the orbit of a 

 comet. The nebulosities of the dense atmospheres of some 

 of these planetoids concealing their disks indicate an incipient 

 change of planetary into cometary bodies. 



Attempts have been made during recent years to discredit 

 the explanation offered by Gibers of the origin of the plane- 

 toids, by assuming that the annulus or convolute of nebular 

 substance failed to resolve itself into a sphere, but was broken 

 up into a number of small bodies. 



There is no inherent improbability in the idea of a nebular 

 convolution resolving itself into a number of discrete spherical 

 bodies, as many of such are to be seen in the convolutions of 

 spiral nebula^ of which M. 100 Comas and M. 74 Piscium are 

 the most striking examples. The convolutions of these 

 nebulse contain nebular stars which are involved symmetri- 

 cally, and follow the curvature of the convolutions. M. 100 

 Comse is further interesting from the fact of its showing- 

 elongated fissions of the convolutions previous to their de- 

 velopment into spherical bodies. Such discrete bodies, 

 revolving in a circular orbit of the same diameter, would, by 

 their mutual attractions, ultimately coalesce to form a single 

 planet, as postulated in my paper in connexion with the 

 contraction of the radius vector of Neptune*. 



As the orbits of all the planets are nearly in the plane of 

 the ecliptic, and also of comparatively small eccentricity, it 

 would become necessary to further assume that ail the rings 

 of discrete bodies should revolve in the same plane of the 

 ecliptic, and in orbits nearly circular as do the other plane- 

 tary bodies ; but Gibers found that Pallas had the large 

 orbital' inclination of 34°* 7, and many others are inclined 

 from 26 to 15 degrees. 



The eccentricities of some of the planetoids are also very 

 large, that of ^Ethra being 0*380, Juno 0*257, and Pallas 

 0*238. The periodic times vary between 7*86 years (Hilda) 

 and 1*75 years (Erosj with the correlated large differences 

 in their mean distances from the sun; Hilda being 3*95 

 astronomical units, and Eros only 1*46 units, which thereby 

 intersects the orbit of Mars, 1*52 units. 



The large differences observable in the elements of the 



* Manchester Memoirs, vol. liv. 1910. Phil. Ma<r. [01 vol. xix. 1910, 

 p. 601. C L 



