256 
GILBERT. 
the theory supposes the internal heat to have been com¬ 
municated.* 
Meteoric Theories .—All other theories which I have been 
able to discover appeal in one way or another to the collision 
of other bodies with the moon’s surface, and for want of a 
better term I shall call them meteoric. If a pebble be 
dropped into a pool of pasty mud, if a raindrop fall upon 
the slimy surface of a sea marsh when the tide is low, or if 
any projectile be made to strike any plastic body with suit¬ 
able velocity, the scar produced by the impact has the form 
of a crater. This crater has a raised rim, suggestive of the 
wreath of the lunar craters. With proper adjustment of 
material, size of projectile, and velocity of impact, such a 
crater scar may be made to have a central hill. Thus scars 
of impact may simulate in many ways the scars of the 
moon’s face, and a number of theories have accordingly been 
broached which agree in regarding the craters as due to the 
bombardment of the moon by projectiles coming from with¬ 
out. As the present study is primarily physiographic, these 
similitudes of form have been considered with great care, 
and it is my belief that all features of the typical lunar crater 
and of its varieties may be explained as the result of impact. 
The special considerations presently to be adduced are along 
this line. 
Long ago it was suggested that the projectiles might have 
been fired from terrestrial volcanoes, but the speed actually 
acquired by the ejecta of volcanoes falls so far short of that 
necessary to carry them beyond the sphere of the earth’s 
attraction that this view is no longer entertained. All other 
suggestions have regarded the material as cosmic. Every 
shooting star records■ by its brief coruscation the collision 
with our atmosphere of a particle of star dust; and though 
the number of these which can be seen by a single observer 
* These statements may do injustice to Peal’s version of the theory, 
which has been given to the world only in abstract. I have not seen his 
fuller exposition in a pamphlet privately printed in India with an edition 
of 100. 
