178 REACTION OF THE INTERIOR OF THE EARTH 



noise similar to a cannonade, unaccompanied by any sen- 

 sible earthquake movement, was heard over a space of 

 10,000 (German) geographical square miles, 15 to a 

 degree. ( 25 ) It is well deserving of notice that when 

 earthquakes are associated with subterranean noise, 

 which is by no means always the case, the loudness of 

 the noise does not increase with the 'strength of the 

 earthquake. The most singular and wonderful pheno- 

 menon of the production of sound of which we have any 

 record is still the subterranean and roaring <s bramidos de 

 Gruanaxuato," from the 9th of January to the middle 

 of the following month in 1784 ; respecting which I 

 was able to obtain the first well assured intelligence 

 from the lips of living witnesses, and from original offi- 

 cial documents. (Vol. I. p. 196, and note 187.) 



The velocity of earthquake propagation at the surface 

 must be supposed to undergo a considerable modifica- 

 tion from the very different densities of rocks, (granite 

 and gneiss, basalt and trachytic porphyry, Jurassic 

 limestone and gypsum,) and of alluvial soil or detritus, 

 passed through by the undulation. It would, however, 

 be exceedingly desirable to learn something as to the ex- 

 treme limits between which the rates of velocity vary. 

 It is probable that the most violent earthquakes are by 

 no means always the most rapidly propagated. The 

 measurements, moreover, do not always apply to the path 

 which the earthquake-waves may have followed. Ac- 

 curate mathematical determinations are greatly wanted. 

 Quite recently, a result has been obtained with great 

 exactness and judicious consideration of all circum- 

 stances, by Julius Schmidt, assistant at the Astrono- 

 mical Observatory of Bonn, on the Rhine earthquake 



