VOL. LI.] PHILOSOPHICAL TRANSACTIONS. 3QI 



was of a firmer texture than what could arise from mere exhalations, whether 

 formed into a sphere, and then burning, or disposed into a kind of train, and 

 consumed by a running fire: for sounds, as far as we know, are either produced 

 by the quick and violent percussions of hard bodies on the air; or by the sudden 

 expansion of an elastic fluid, after being condensed within some solid substance. 

 The noise occasioned by the motion of electrical matter is, perhaps, the only 

 exception; but we have no reason to imagine, that this was at all concerned in 

 the present case. There seems to be the more ground for believing this body 

 was solid, at least that its surface was so, from finding, that after the violent 

 explosion, it still retained its form ; a circumstance that could hardly take place 

 if the meteor had consisted of nothing but vapours. We may therefore pre- 

 sume, that the burning matter found vent through a hard crust by certain aper- 

 tures, which either might have been there invisible, or unobserved. All I can 

 say in support of this conjecture is, that, by the Memoirs of the Academy of 

 Bologna, we find a meteor appeared in Italy in the year 1719> lower in the air 

 than that we have now been treating of, and in which, it is pretended, four 

 several chasms were distinguished, each emitting smoke,-|- To these arguments 

 for the solidity of this body, we may add its extreme velocity, and the intensity 

 of the light: which are likewise circumstances more conformable to a heavy and 

 solid substance, than to one formed of exhalations only. 



On the whole, Dr. P. believes it will appear, that these accounts are not fa- 

 vourable to the prevailing hypothesis about the formation of such bodies, which 

 makes them to consist of certain sulphureous vapours arising from the earth: 

 for, besides what has been urged above. Dr. Halley has shown, " That at the 

 height of 41 miles, the air is so rarefied as to take up 3000 times the space it 

 occupies on the surface of the earth; and that, at 53 miles high, it would be 

 expanded above 30000 times : but thinks it is probable, that the utmost power 

 of its spring cannot exert itself to so great an extension ; and that no part of 

 the atmosphere reaches above 45 miles."J This being the case, how can we 

 suppose any such vapours to rise to the height of QO or 100 miles, where the air 

 must be so many millions of times rarer than what we breathe? 



Some have been of opinion, that these fiery meteors are only a kind of light- 

 ning, at greater heights than common ; forming their notion on the velocity of 

 those balls of fire, and on the sound accompanying them, so much resembling 



* M. Saluce has lately shown, by some curious experiments, that such substances as gunpowder 

 and the pulvis fulminans, have a detonation in proportion to the rapidity with which the internal air 

 is separated, and to the resistance of the external air. See Miscell. Taurinens, torn. 1. — Orig. 



f Apparebant in eo hiatus seu voragines quatuor fumum exhalantes. Instit. et Acad. Bonon. torn. 

 1, p. 285. — Orig. 



X Phil. Trans. N° 181, p. 104. Abridg. vol. iii. 800. 



