262 
GILBERT. 
way. One of these groups is arranged in the form of a ring 
and encircles the planet Saturn. This ring is broad and 
thin, and all parts of it lie nearly in one plane. The 
meteors which constitute it are so numerous that portions 
of the ring appear continuous and solid. They are too 
small to be individually perceived, but there can be little 
question that they all travel about the planet in a system of 
parallel orbits and with correspondingly adjusted velocities. 
It is my hypothesis that before our moon came into exist¬ 
ence the eartli was surrounded by a ring similar to the 
Saturnian ring; that the small bodies constituting this ring 
afterward gradually coalesced, gathering first around a large 
number of nuclei, and finally all uniting in a single sphere, 
the moon. Under this hypothesis the lunar craters are the 
scars produced by the collision of those minor aggregations, 
or moonlets, which last surrendered their individuality. 
This change of conception yields a material difference in 
the law of the directions in which minor bodies approach 
the moon, the difference depending on the fact that all the 
minor bodies colliding with the greater body have initial 
orbits lying approximately in the same plane. To render 
this clear it is necessary to amplify the statement already 
made with reference to the predominant angle at which 
cosmic meteors encounter the surface of the moon. Their 
velocities are so high, as compared with the acceleration 
due to lunar attraction, that their courses in the vicinity of 
the moon may, without sensible error, be regarded as straight. 
The angle at which each one strikes the moon’s surface de¬ 
pends upon the nearest distance of its produced orbit from 
the moon’s center, and is entirely independent of the direc¬ 
tion from which it approaches. We may therefore simplify 
the discussion of incidence angles by assuming that the 
meteors all come from the same direction and move along 
parallel lines. The number of meteorites being indefinitely 
large and their distribution entirely independent of the 
moon, we may for this purpose conceive them as an evenly 
distributed rain, of which the moon receives a certain por- 
