J04 COSMOS. 



be found, since the region in which it fell is now become 

 so easy of access to European travellers. The huge aerolite 

 which in the beginning of the tenth century fell into the river 

 at Narni, projected between three and four feet above the sur- 

 face of the water, as we learn from a document lately dis- 

 covered by Pertz. It must be remarked that these meteoric 

 bodies, whether in ancient or modern times, can only be re- 

 garded as the principal fragments of masses that have been 

 broken up by the explosion either of a fire ball or a dark 

 cloud. 



On considering the enormous velocity with which, as has 

 been mathematically proved, meteoric stones reach the earth 

 from the extremest confines of the atmosphere, and the 

 lengthened course traversed by fire balls through the denser 

 strata of the air, it seems more than improbable that these 

 metalliferous stony masses, containing perfectly-formed crys- 

 tals of olivine, labradorite, and pyroxene, should in so short 

 a period of time have been converted from a vaporous con- 

 dition to a solid nucleus. Moreover, that which falls from 

 meteoric masses, even where the internal composition is 

 chemically different, exhibits almost always the peculiar cha- 

 racter of a fragment, being of a prismatic or truncated pyra- 

 midal form, with broad somewhat curved faces, and rounded 

 angles. But whence comes this form, which was first recog- 

 nised by Schreiber, as characteristic of the severed part of a 

 rotating planetary body ? Here, as in the sphere of organic 

 life, all that appertains to the history of development remains 

 hidden in obscurity. Meteoric masses become luminous and 

 kindle at heights which must be regarded as almost devoid of 

 air, or occupied by an atmosphere that does not even contain 

 10 0*0.60 part of oxygen. The recent investigations of Biot, on 

 the important phenomenon of twilight,* have considerably 



* Biot, Traite d 'Astronomie physique (3eme 6d.), 1841, t. i. pp. 149, 

 177, 238, 312. My lamented friend Poisson endeavoured, in a singular 

 jnanner, to solve the difficulty attending an assumption of the spon- 

 taneous ignition of meteoric stones at an elevation where the density 

 of the atmosphere is almost null. These are his words : " It is difficult 

 to attribute, as is usually done, the incandescence of aerolites to friction 

 against the molecules of the atmosphere, at an elevation above the earth 

 where the density of the air is almost null. May we not suppose that 

 the electric fluid, in a neutral condition, forms a kind of atmosphere, 

 extending far beyond the mass of our atmosphere, yet subject to ter 



