172 COSMOS. 



that when earthquakes are combined with noises, which is 

 by no means constantly the case, the strength of the latter 

 does not at all increase in proportion to that of the former. 

 The most singular and mysterious phenomenon of subter- 

 ranean sound is undoubtedly that of the brainidos de Gua- 

 naxuato, which lasted from the 9th of January to the middle 

 of February, 1784, regarding which I was the first to col- 

 lect trustworthy details from the lips of living witnesses, 

 and from official records (Cosmos, vol. i., p. 20 ( J). 



The rapidity of the propagation of the earthquake upon 

 the surface of the earth must, from its nature, 1 c modified in 

 many ways by the variable densities of the solid rocky strata 

 (granite and gneiss, basalt and trachytic porphyry, Jurassic 

 limestone and gypsum), as well as by that of the alluvial 

 soil, through which the wave of commotion passes. It 

 would, however, be desirable to ascertain once for all with 

 certainty what are the extreme limits between which the 

 velocities vary. It is probable that the more violent com- 

 motions by no means always possess the greatest velocity. 

 The measurements, moreover, do not always relate to the 

 same direction which the waves of commotion have followed. 

 Exact mathematical determinations are much wanted, and 

 it is only at a very recent period that a result has been ob- 

 tained with great exactitude and care from the Ehenish 

 earthquake of the 29th of July, 1846, by Julius Schmidt, 

 assistant at the Observatory of Bonn. In the earthquake 

 just mentioned the velocity of propagation was 14,956 geo- 

 graphical miles in a minute, that is, 1466 feet in the second. 

 This velocity certainly exceeds that of the waves of sound in 

 the air ; but if the propagation of sound in water is at the 

 rate of 5016 feet, as stated by Colladon and Sturm, and in 

 cast-iron tubes 11,393 feet, according to Biot, the result 

 found for the earthquake appears very weak. For the 

 earthquake of Lisbon, on the 1st of November, 1755, Schmidt 

 (working from less accurate data) found the velocity between 

 the coasts of Portugal and Holstein to be more than five 

 times as great as that observed on the Rhine, on the 29th of 

 July, 1846. Thus, for Lisbon and Gliickstadt (a distance 



let upon sonorous waves in the earth and sonorous waves in the air 

 occur in the Brit. Assoc. Report, 1850, p. 41-46, and in the Admiral- 

 ty Manual, 1849, p. 201 and 217. The animals which in tropical 

 countries are disquieted by the slightest commotions of the earth 

 sooner than man are, according to my experience, fowls, pigs, dogs, 

 asses, and crocodiles (Caymans) ; the latter suddenly quit the bottom 

 of the rivers. 



