144 NINTH A.NNUAL REPORT OF 



included within the space of 180 degrees ; while only eleven are left 

 for the remaining 180 degrees of the zodiac. A similar remark ap- 

 plies to the position of the perihelia. In the first half of the zodiac 

 we find twenty-five perihelia, while only eight remain for the other 

 half of the circumference. We could not have anticipated any such 

 bias in the orbits if they had always been entireh' independent of each 

 other. 



4. But the most striking peculiarity of these orbits is, that they all 

 lock into one another like the links of a chain, so that if the orbits are 

 supposed to be represented materially as hoops, they all hang together 

 as one system. The orbits of Hygeia and Themis being the largest 

 of all the orbits, completely enclose nearly all of them, and lock into 

 but a small number ; while the orbits of Massilia, Astrasa, Pallas, &c., 

 lock into nearly all of the orbits, so that if we take hold of the orbit of 

 H3'geia, which we fancy to be a material hoop, it will support the orbits 

 of Iris, Thalia, Calliope, and two or three others, while these in turn 

 lock into and support all the rest. Indeed, if we seize hold of any 

 orbit at random, it will drao- all the other orbits along- with it. This 

 feature, by itself, sufficiently distinguishes the asteroid orbits from all 

 the other orbits of the solar system. 



If we reject the theoiy that these asteroids were originally united 

 in one solid body, it seems, nevertheless, difficult to avoid the conclusion 

 that similar causes have operated in determining the orbits of this zone 

 of planets. It is impossible to assign any cause for these resemblances 

 without adopting some theory respecting the origin of the solar s^^stem. 

 Tiie theory of gradual condensation, as developed by Laplace in the 

 nebular hypothesis, affords at least a plausible explanation of these 

 phenomena. 



, Laplace supposes that the matter composing the bodies of our solar 

 system, originall}' existed in the condition of an immense nebula, ex- 

 tending beyond the limits of the most distant planet — that this nebulous 

 mass had an exceedingly elevated temperature, and a slow rotation on 

 its axis — that the nebula gradually cooled; and as it contracted in di- 

 mensions, its velocity of rotation, according to the principles of me- 

 chanics, increased, until the centrifugal force arising from the rotation, 

 became equal to the attraction oi' the central mass for the exterior zone, 

 when this zone necessarily became detached from the central mass. As 

 the central mass continued to contract in its dimensions, and its velocity 

 of rotation continued to increase, the centrifugal force again became 

 equal to the attraction of the central mass for the exterior zone, and a 

 second zone was detached. Thus a number of zones of nebulous 

 matter were successively detached until, by gradual condensation, the 

 central mass became of comparatively small dimensions and great 

 density. 



The zones thus successively detached would form concentric rings 

 of vapor, all revolving in the same direction round the sun. If the 

 particles of each ring continued to condense without separating from 

 each other, they would ultimately form a liquid or a solid ring. But 

 generally each ring of vapor would break up into separate masses, 

 revolving about the sun with velocities slightly different from each 

 other. These masses would assume a spheroidal form : that is, they 



