THE MOON. 155 



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 lire, 

 and elastic eruptive vapors, and even to an ebullition in 

 eruptive bubbles." Experiments with thickened boiling lime 

 solutions appeared to him to confirm his opinion ; and the cir- 

 cumvallations, with their central mountains, were at that time 

 already compared with " the forms of iEtna, the Peak of 

 TeneriHe, Hecla, and the Mexican volcanoes described by 

 Gage."* 



One of the annular plains of the Moon reminded Galileo, 

 as he himself relates, of the configuration of countries entirely 

 surrounded by mountains. I have discovered a passagef in 

 which he compares these annular 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 diameter of from 

 100 to 120 geographical miles. $ On the contrary, the real an- 

 nular mountains scarcely exceed 8 or 12 miles in diameter. 

 Conon in the Apennines is 8 ; and a crater which belongs to 

 the shining region of Aristarchus is said to present a breadth 

 of only 25,576 feet, exactly the half of the diameter of the 

 crater of Rucu-Pichincha, in the table-land of Quito, meas- 

 ured trigonometric ally by myself. 



Since we have in this place adhered to comparisons with 

 well-known terrestrial phenomena and relations of magnitude, 

 it is necessary to remark that the greater part of the plains 

 and annular mountains of the Moon are to be considered in 

 the first place as craters of elevation, without continuous 

 phenomena of eruption in the sense of the hypothesis of Leo- 

 pold von Buch. What, according to the European standard, 



* Robert Hooke, Micrographia, 1667, Obs. lx., p. 242-246. " Theso 

 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 am- 

 bient surface of the Moon, so has it left a hole or depression in the mid- 

 dle, proportionably lower." Hooke says of his experiment with boil- 

 ing 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 vapors, 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 " 



t Cosmos, vol. ii., p. 319, note. 



% Beer and Madler, p. 126. Ptolemseus is 96 miles in diameter 

 Alphons and Hipparchus. 76 miles. 



