ON THE FACTS OF EARTHQUAKE PHANOMENA,. 43 
] ᾿ strata, falling in of rocks, grinding of masses over each other, or the reper- 
᾿ς eussion produced by steam, evolved by the heat of molten matter under the 
earth or sea, and again suddenly condensed by being driven into contact 
with cold water; in any of these cases if the centre of impulse producing the 
noise, be distant from that producing the shock, or if the two waves, 7. 6. that 
of sound and of shock, arrive to the ear through different media, they will 
arrive at different moments. So if the impulse be extended along a line of 
impact passing away from the listener, he will hear a prolonged sound from 
a single blow, producing perhaps but a single shock. The general rule, 
however, ἃ priori, is that the sound-wave and the earth-wave of shock travel 
at the same speed through the same formations, and if they arise from one 
common impulse will generally reach the ear and observer at the same mo- 
ment; and accordingly this is by far the most usual case recorded; but 
innumerable perturbations and complications of this may and do take place, 
many of which I have remarked upon in my paper on the Dynamics of 
Earthquakes (Trans. R. I. Acad.), and for brevity here I pass them over as 
easily predicted by those versed in acoustics. 
In most earthquakes perhaps, certainly in very many, a sound is heard 
before the great shock, and usually a vibratory jar felt also. The earth- 
᾿ς quake ) Tiffliz, in Georgia, of 29th of January 1818, was so. (Quart. Journ. 
vol. vii. 
Count Mercate, in his account of the earthquake of 20th of December 
_ 1820, at Zante, says the sound was heard before the shock was felt. (Quart. 
Journ. vol. xviii.) 
᾿ς Hamilton says of the Calabrian earthquake, “ All agreed that every shock 
_ seemed to come with a rumbling noise from the westward, beginning with 
_ the horizontal and ending with the vorticose motion.” (Phil. Trans. vol. 
__ Ixxii.) This does not apply to the great shock of the 5th of February, “ which 
_ was from below upwards.” 
In the Chili earthquake of 1822-23, the explosive sound and great shocks 
seem to have arrived simultaneously. (Mrs. Graham, Geol. Trans. vols. i. ii. 
ser. 2. p. 413.) 
Dolomieu says, the Calabrian shocks “ were preceded by a loud subterra- 
neous noise like thunder, which was renewed every shock,” speaking not 
however of the great upward shocks. “ This great shock,” he says (5 Feb.), 
_ “occurred without the prelude of any slighter shocks, without any notice 
whatever, as suddenly as the blowing up of a mine.” “Some however 
pretend that a muffled interior noise was heard almost at the same 
moment.” 
The great Lisbon earthquake “began with a noise like the rumbling of 
_ earriages, which grew gradually louder until it equalled the loudest artillery, 
_ and then the first great shock occurred.” (Phil. Trans. vol. xlvi. xlix. lviii.) 
___In some very great earthquakes it should be remarked that a very loud 
__ noise has been heard a very considerable lapse of time after the shock. 
_ Thus at Quito and Ibarra the great noise (el gran ruido) was heard eighteen 
or twenty minutes after the shock. At Lima and Callao, in the great 
earthquake of October 1746, the subterraneous peal of thunder was heard 
at Truxillo fifteen minutes after the shock. These great noises could scarcely 
have been due to the same impulse that produced the original shock, but 
more probably to a subsequent one, whose shock was delivered in a different 
| direction to the first, and hence not felt at the places where the sound was 
heard, which may have reached them indirectly and through the air. 
| The time of transit of the sound-wave will manifestly differ, whether it 
reach the ear through the sea or through the solid land; in the former case 
