!8 9 3. THE SURFACE OF THE MOON. 51 



of the target was softer than the portion at the surface, the uplift 

 consequent on the production of the hollow was only partially 

 localised about its periphery, the remaining part being widely 

 distributed through the floor of the softer material below. The last 

 difficulty is connected with the circular contours of the craters. The 

 predominant direction of the swiftly moving meteoric bodies approach- 

 ing from all directions would be about 45 , and the scars produced 

 by collision would be predominantly oval instead of predominantly 

 circular. Proctor suggested that immediately after the shock of the 

 collision there might be an elastic return to a circular form (" The 

 Moon," London, 1873, p. 346) ; but to account for this peculiarity of 

 form of the lunar craters, Gilbert advances a " Moonlet Theory," 

 assuming that before our moon came into existence the earth was 

 surrounded by a ring of meteors similar to that possessed by Saturn, 

 and 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 these minor 

 aggregations or moonlets, which last surrendered their individuality. 

 This theory is fully explained and worked out in the address, to 

 which the reader must be referred for details. 



Professor Gilbert then discusses the " arched floors " of some of 

 the larger craters, and the distribution and overlap and interference 

 of the craters themselves ; and then turns to the question of lunar 

 sculpture. The sculpturing of the Mare Imbrium has particularly 

 engaged his attention, and he points out that the trend of the sculp- 

 ture lines and a peculiar softening of the minute surface configuration, 

 as though a layer of semi-liquid matter had been overspread (which 

 he believes to be the fact, the deposit having obliterated the smaller 

 craters and partially filled some of the larger), indicate that a collision 

 of exceptional importance occurred in the area of the Mare Imbrium, 

 and that one of the results was the violent dispersion in all directions 

 of a deluge of material — solid, pasty, and liquid. The effect of this 

 " splash " has been traced by Gilbert goo-1,000 miles in one direction, 

 and he estimates the volume of the projectile to have equalled a 

 sphere 80-100 miles in diameter. A sketch-map is given, showing the 

 trend of the lines of sculpture, and the broken and notched rim of the 

 Mare Imbrium. 



Referring to the "furrows" seen on the moon's surface, so re- 

 markably straight in general, one of which is calculated to be 

 187 miles long, 10-25 miles wide, and 11,000 feet deep, it is suggested 

 that these are the tracks left by solid moonlets, whose orbits at the 

 instant of collision were nearly tangent to the surface of the moon. 

 It is curious that when plotted on the chart of the moon's face, Pro- 

 fessor Gilbert found that more than half of them accorded in direction 

 with the trend lines of the Imbrium outrush, and he is led to suggest a 

 connection between them and the scattered fragments dispersed by 



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