XXVI NOTES. 



since abandoned, opinion of the existence of active volcanoes in the moon, 

 where air and water are absent, caused the public to confound two things 

 extremely different, viz. a mathematical possibility and a physical probability. 

 Olbers, Brandes, and Chladni, considered that, in the relative velocity of 

 4 to 8 German, or 16 to 32 English miles, with which balls ot fire and 

 shooting stars enter our atmosphere, they found a refutation of lunar origin. 

 According to Olbers, the initial velocity required to reach the earth, with- 

 out taking into account the resistance of the atmosphere, would be 7780 

 French feet in a second ; according to Laplace, 7377 ; according to Biot, 

 7771 ; and according to Poisson, 7123. Laplace calls this an initial velo 

 city only five or six times greater than that of a cannon-ball ; but Olbers has 

 shewn, " that with an initial velocity of 7500 to 8000 French feet in a second, 

 meteoric stones would arrive at the surface of the earth with a velocity only 

 of 35000 feet (1'53 German geographical miles): now as the mean mea- 

 sured velocity per second of meteoric stones is 5 German geographical miles, 

 or abo^e 114000 feet per second, it follows that the initial velocity at the 

 surface of the moon should be almost 110000 feet, or 14 times greater than 

 that assumed by Laplace." (Olbers, in Schum. Jahrb. 1837, S. 52 58; 

 tud in Gehler's neuem Physik. Worte.'buche, Bd. vi. Abth. 3, S. 21292186). 

 It is true, that if we could assume volcanic forces to be active at the surface 

 of the moon at the present time, the absence of atmospheric resistance would 

 give to the projectile force of lunar volcanoes an advantage over that of our 

 terrestrial volcanoes ; but even in respect to a measure of the latter force, data 

 on which we can depend are extremely deficient, and it is probable that it has 

 been greatly over-estimated. A very accurate observer of the pheenomena of 

 Etna, Dr. Peters, found the greatest velocity of any of the stones which he 

 saw ejected from the crater only 1250 feet in a second ; observations on the 

 Peak of Tenenffe, in 1798, gave 3000 feet. Although Laplace, at the end of 

 his work (Expos, du Syst. du Monde, ed. de 1824, p. 3 ( J ( i), says respecting 

 aerolites, " que selon toutes les vraisemblances, elles viennent des profoudeurs 

 de 1'espace celeste ;" yet we see from another passage (Chap. vi. p. 233), that, 

 being probably unacquainted with the enormous planetary velocity of meteoric 

 stones, he turned with a decree of preference to the hypothesis of a lunar 

 origin, always, however, premising the assumption that the stones projected 

 from the moon " deviennent des satellites de la terre, decrivant autour d'elle 

 une orbite, plus ou moins allongee, de sorte qu'ils n'attcignent 1' atmosphere de 

 la terre qu'apres plusieurs et meme un tres grand nombre de revolutions." As 

 an Italian at Tortona conceived that aerolites came from the moou, so some 



