■911] SOLAR SYSTEM BEYOND NEPTUNE. 267 



absorption of which also increased the masses of the planets 

 enormously. 



4. If one asks for ocular evidence that the planetary bodies have 

 been in collision with smaller masses, this evidence is found in the 

 phenomena shown in the face of the moon, which was formerly an 

 independent planet, and is so small a globe as never to have devel- 

 oped water or atmosphere ; so that it is a kind of hermetically sealed 

 celestial museum, so near us in space that it serves for the illustra- 

 tion of the process of absorption and capture in cosmogony. The 

 type of collisions visibly illustrated by the dents in the moon's face 

 necessarily have occurred with all the planets ; but the moon as our 

 nearest planetary neighbor alone enables us to study the process of 

 accretion by collisions with bodies of all sizes, from particles to 

 satellites as large as twenty miles in diameter. 



5. The obvious deposits of dust over the older lunar craters give 

 them an aspect of great age, and in many cases the outlines of the 

 craters are practically obliterated. In other cases newer craters are 

 formed over the older ones ; so that we can certainly infer by direct 

 observation that the moon has been built up by accretion, dust being 

 gathered in to be deposited over dust, and crater over crater. This 

 is the same process which we see at work on the earth, except that 

 the meteorites now swept up by our planet are generally small and 

 consumed in the air before reaching the earth. 



6. Since the planets were begun as independent nuclei in our 

 nebula, and since augmented by the gathering together of an infinite 

 number of small bodies, such as comets, the matter of planets and 

 comets must necessarily be the same, for they are common products 

 of our ancient nebula. The planets have been built up by the gather- 

 ing in of satellites, comets and smaller particles of cosmical dust. 



7. Xow we have pointed out that Neptune's orbit is too round 

 for it to be the outermost of the planets of the solar system. If the 

 resisting medium was dense enough at that great distance to produce 

 such extreme circularity in the motion of Neptune, there was 

 enough of the nebulosity beyond that planet to make several more 

 planets of comparatively large size. Thus it is certain that our 

 system does not terminate at Neptune, but extends on almost inde- 

 finitely. It is probable that in time we may be able to discover 



