52 NATURAL SCIENCE. j ULY , 



the collision in that district. The anomalous plateau of Wargentinis 

 considered, but no definite suggestion is made, although clues are not 

 wanting, for the whole neighbourhood abounds in evidence of flooding. 



With regard to the " white streaks," which are quite independent 

 of any surface configuration, and which usually radiate from some 

 crater itself lined with a similar white matter, Professor Gilbert 

 quotes an unpublished suggestion of William Wiirdemann of Wash- 

 ington, which is as ingenious as it is simple. Wiirdemann writes to 

 Dr. B. A. Gould as follows : — " The most remarkable appearance on 

 the moon, for which nothing on earth furnishes an example, is pre- 

 sented by those immense radiations from a few of the larger craters — 

 perfectly straight lines, as though marked with chalk along a ruler- 

 starting from the centre of the crater and extending to great distances 

 over every obstruction. My explanation is that a meteorite, striking 

 the moon with great force, spattered some whitish matter in various 

 directions. Since gravitation is much feebler on the moon than with 

 us, and atmospheric obstruction of consequence does not exist, the 

 great distance to which the matter flew is easily accounted for." 

 Gilbert remarks that this theory accounts for the straightness of the 

 rays, for their vanishing edges and ends, for their independence of 

 topography, for their relation to craters, for the whiteness of the 

 associated craters, and for the nimbus in which the rays sometimes 

 unite close to the crater. It further explains the white crests of 

 many grey craters, for peaks would intercept more than their share 

 of the horizontal shower. The difficulties and suggestions of this 

 theory are fully dealt with by Professor Gilbert, and he is inclined 

 to believe the white matter to be a readily fusible solid. 



Summing up the various theories in his retrospect, Professor 

 Gilbert says : — the impact theory applies a single process to the forma- 

 tion of craters (excepting only the rill pits), correlating size variation 

 with form variation in a rational way. Specialised by the assumption of 

 an antecedent ring of moonlets, it accounts also for the great size of 

 many craters. It brings to light the history of a great cataclysm, 

 whose results include the remodelling of vast areas, the flooding of 

 crater cups, the formation of irregular "maria," and the conversion of 

 mere cracks to rills with flat bottoms. It explains the straight 

 valleys and the white streaks. In fine, it unites and organises as a 

 rational and coherent whole the varied and strange appearances 

 whose assemblage on our neighbour's face cannot have been 

 fortuitous. 



The address concludes with a brief discussion of the growth 

 and age of the moon. Professor Gilbert believes that during the 

 whole period of growth the surface lost heat by radiation, but 

 the process of growth cannot have been slow enough to permit the 

 concurrent dissipation of all the impact heat. Liquefaction was a 

 local and temporary surface phenomenon, and there is little evidence 

 of the wrinkling, which, theoretically, should result from the adjust- 



