AND ITS INHABITANTS 27 



miles in increasing numbers down to 15 to 20 miles, the limit 

 of telescopic visibility. 



At some diameter below the limit of visibility In the tele- 

 scope, although the number may be increasingly great, the 

 summation of their masses must begin to fall off, since other- 

 wise the combined bulk would produce a perceptible glow in 

 the sky. Furthermore, Leverrier demonstrated from the 

 limited perturbations of Mars in its orbit that the whole 

 amount of matter distributed between the orbits of Mars and 

 Jupiter cannot exceed about one-fourth of the mass of the 

 earth. It may be less. In fact, later calculations limit it to 

 less than one-hundredth of the mass of the earth. The rate 

 of increase in numbers in the smaller visible sizes suggests in 

 connection with the limitation in aggregate mass that a con- 

 siderable part, perhaps a larger part of the matter, is not in 

 dust-like or molecular form but is in fragments of appreciable 

 size ranging up to some miles in diameter. These masses, 

 owing to their small diameters and weak gravitative force, 

 would possess almost no power to grow by accretion. They 

 must retain almost the original state of the nebula, or better, 

 the meteoritic swarm, and are perhaps as likely to have suf- 

 fered occasional shattering and scattering by impact as to have 

 grown from a lower order of size. Their evidence favors the 

 hypothesis that the scattered matter which was added to the 

 nucleus to form the earth was largely of such size that the 

 individual planetoids would have plowed through a primordial 

 atmosphere and ocean, if such existed, and have penetrated 

 beneath the surface of the liquid or solid body below. The 

 energy of impact from dust-like material would be absorbed 

 at the surface and, as heat, quickly radiated into space. The 

 accretion of dust would favor the growth of an earth solid 

 throughout. Larger masses would, on the other hand, carry 

 the energy of impact into the earth. They would not strike 

 with the high velocities of the meteors which collide with the 



