50 NATURAL SCIENCE. july. 



judge Peal's views, which have been circulated in a pamphlet, of 

 which loo copies were privately printed, and therefore practically 

 unpublished and inaccessible. 



All other theories which have been put forward appeal in some 

 way or other to the collision of other bodies with the moon's surface, 

 and Professor Gilbert groups them together under the heading 

 " Meteoric." Proctor apparently was the first to definitely suggest 

 a " meteoric " theory in 1873 and 1878. " Asterios " and Meyden- 

 bauer have also advanced similar views. The " meteoric " theory is 

 thus defined in Gilbert's paper : — If a pebble be dropped into a pool 

 of pasty mud, if a rain-drop falls 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 suitable 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, and it is Professor Gilbert's belief that 

 all features of the typical lunar crater and of its varieties may be 

 explained as the result of impact. His arguments may be summed 

 up in the following sketch. The shooting star records by its brief 

 coruscation the collision with our atmosphere of a particle of star 

 dust ; and although they are generally very minute in size, a few 

 survive after passing through an atmosphere and reach the earth as 

 aerolites weighing ounces, pounds, or occasionally tons. It is also an 

 ascertained fact that these bodies are spreading in countless myriads 

 through space in all directions. As the moon is either without atmo- 

 sphere, or has one of extreme tenuity, the impact of one of these 

 bodies may have an important effect on the surface. As it is incredible 

 that even the largest meteors of which we have direct knowledge 

 should produce scars comparable in magnitude with even the smallest 

 of the visible lunar craters, advocates of meteoric theories have 

 assumed that at some earlier period the meteors encountered by our 

 solar system were of greater size than now, and as no evidence has 

 been found that the earth was subjected to a similar attack, there is 

 assigned to the lunar bombardment an epoch more remote than are 

 the periods of geological history, any similar scars produced on the 

 earth having been obliterated. 



Professor Gilbert does not assume that the surface of the moon 

 was necessarily soft. Rigidity and plasticity are not absolute terms, 

 but relative. But he suggests that the heat developed by the sudden 

 arrest of a fragment of matter travelling at the rate of 45 miles per 

 second might serve not only to melt the fragment itself, but also to 

 liquefy a considerable tract of the rock mass by which its motion 

 was arrested. The difficulty of the relation of the volume of the rim 

 to the capacity of the hole is carefully considered, and experiment 

 proved that when target and projectile were of uniform consistency 

 throughout, there was no defect of rim ; but when the general mass 



