90 THE GREAT LUNAR CRATER TY(]HO. 



Monte Siiinna would appear as insignificant little hills if they were 

 dropped into the center of the crater of Tycho, whose ring' wall towers 

 to a height of 17,000 feet above the plain it incloses. 



Robert Hooke compared the lunar craters to the cup-sha[)ed pits 

 formed on the surface of boiling mud by escaping vapor, and the idea 

 has been a fascinating one to many minds since his day, though it needs 

 but little consideration to recognize that bubbles or blisters formed iu 

 a plastic material on a scale corresponding with that of the lunar craters 

 would rapidly siidc down and be obliterated. 



Mr. S. E. Peal has ingeniously advocated a theory which seems to 

 me almost equally untenable. He assumes that the lunar surface con- 

 sists entirely of ice, and that the craters and pit-like depressions are 

 due to the action of hot springs which have not flowed continuously, 

 but that water has from time to time issued from vents in the soil, and 

 has melted the ice above the vent. The water is then supposed to have 

 flowed back to the warm interior of the moon, taking with it a part of 

 the surface ice that has been melted, and by a series of such ebbs and 

 flows Mr. Peal conceives the terraced walls of the lunar craters to have 

 been built up above the level of surrounding plains. Putting on one 

 side the difficulty of conceiving of nearly x>erpendicular ice cliffs of 

 17,000 to 20,000 feet high, standing for ages without flowing down as 

 glaciers to the plains at their feet, we have to account for the fact that 

 the lunar plains and the floors of the deeper lunar craters are generally 

 of a much darker tint than the higher ground upon the moon, while if 

 the whole of the lunar surface were composed of ice and snow there 

 would be no reason for such a difference of tint, unless, as Mr. I'eal 

 suggests, the lunar ijlains are surfaces of virgin ice while the moun- 

 tains are formed of snow. But virgin ice would reflect the light of the 

 sun specularly, and in the equatorial and tropical parts of the moon 

 from which the sun's rays could be specularly reflected to us there are 

 no traces of such specular reflection. The theory also fails to account 

 for the small craters frequently found on the rims of large craters and 

 on the sloping sides of mountains. Such small craters are far above 

 the assumed rock surface of the moon, and warm water issuing from 

 them would flow down the sides of the mountains leaving marked traces 

 of its flow. The meteoric theory of the formation of lunar craters has 

 also had many advocates. It is alleged that if a pebble be dropi)cd 

 into mud the scar produced has a raised rim and a central hill, which 

 resembles a lunar crater. Even Mr. Proctor had an inclination for this 

 theory. At page 346 of his book on the moon he says: " So far as the 

 smaller craters are concerned, there is nothing incredible in the suppo- 



schatka ("Volcanoes," second edition, London, 1862, p. 457;; (4) an imperfect cra- 

 ter cirque on Mauritius, mentioned by Cliarles Darwiu, is mapped (Admiralty) as 

 about 15 by 16 miles in extent; (5) the crater walls surrounding Lake Bolesua, 

 Italy, are mapped as 11 by 9 miles in extent ; (6) the crater containing Lake Man iuju, 

 Sumatra, is mapped (Rectus) as 15 by 7 miles in extent. 



