496 COSMOS. 



the walled plains which frequently surround central moun- 

 tains; in the large annular mountains and their craters (22 

 are counted close together in Bayer, and 33 in Albategnius}, 

 must have early induced a deep-thinker like Robert Hooke, 

 to ascribe such a form to the reaction of the interior of the 

 Moon upon the exterior. " The action of subterranean fire, 

 and elastic eruptive vapours, and even to an ebullition in 

 eruptive bubbles. Experiments with thickened boiling lime 

 solutions appeared to him to confirm his opinion; and the 

 circumvallations, with their central mountains, were at that 

 time already compared with " the forms of -<Etna, the peak 

 of Teneriffe, Hecla, and the Mexican volcanos described by 

 Gage." a 



One of the circular plains of the Moon reminded Galileo, 

 as he himself relates, of the configuration of countries 

 entirely surrounded by mountains. I have discovered a 

 passage* 8 in which he compares these circular plains of the 

 Moon with the great inclosed basin of Bohemia. Many of 

 the plains are in fact not much smaller; for they have a 



44 Robert Hooke, Micrographia, 1667, Obs. Ix. pp. 242- 246. 

 " These seem to me to have been the effects of some motions 

 within the body of the Moon, analogous to our earthquakes, 

 by the eruption of which, as it has thrown up a brim or ridge 

 round about, higher than the ambient surface of the Moon, 

 so has it left a hole or depression in the middle, proportion- 

 ably lower." Hooke says of his experiment with boiling 

 alabaster: that " presently ceasing to boyl, the whole surface 

 will appear all over covered with small pits, exactly shaped 

 like those of the Moon. The earthy part of the Moon has 

 been undermined, or heaved up by eruptions of vapours, and 

 thrown into the same kind of figured holes as the powder of 

 alabaster. It is not improbable also, that there may be 

 generated within the body of the Moon, divers such kind of 

 internal fires and heats as may produce exhalations." 



tt Cosmos, vol. ii. p. 701, note 



