ON THE FACTS OF EARTHQUAKE PHZNOMENA, 29 
2 
ἃ minutes ;” “the shock continued nearly an hour,” and other such vague ex- 
pressions. The abuse of language here consists in almost every author using 
the words, duration of the shock, as synonymous with the whole period of 
motion, comprehended from any one commencement to the next great pause 
during the occurrence of the whole earthquake. To be able at all clearly to 
comprehend these narratives, it is necessary to bear in mind that the word, 
shock, is properly limited to the single motion due to a single impulse; that 
this motion occupies an extremely short time in passing a given station, and 
that when “ the shock lasting some minutes,” &c. is spoken of, it means that 
for some minutes there was a succession of these motions with short or 
variable intervals between them ; ὃ. 6. ἃ great number of shocks in quick suc- 
cession. Hence we find that 
12th. The earth-wave or shock is a motion of great velocity and 
occurring during a very short moment of time at any given 
spot. 
It varies indefinitely however in force and in extent of motion; sometimes 
it amounts to a concussion like the blowing up of a mine at a great depth 
under one’s feet; at other times it 15 ἃ mere vibration scarcely to be felt, like 
that produced by a carriage running over a distant pavement; yet these are 
but degrees of the same thing. So again the rapidity with which the shocks 
succeed each other varies. Sometimes a single powerful shock is felt alone, 
or but two or three are felt in pretty rapid succession, and then a period of 
complete or of comparative tranquillity occurs, during which the shocks are 
so reduced in power as to require attention to perceive them; and in this 
ease they often recur with such rapidity as to convey to the observer the 
idea of a vibration or continuous jar, and this often along with the roll of the 
greater wave-like shock. 
It has been ascertained that sixteen vibrations per second, or 960 in a 
minute, is about the limit at which the ear distinguishes between a continuous 
sound or tone, and a regularly recurring noise or jar. I am not aware that 
any information exists as to the relative sensibility to recurrent vibrations of 
the ear, and of the nerves of feeling generally ; but assuming them to be about 
_ the same, it follows, that when the number of shocks per second is about six- 
teen, nothing will be felt but a continuous vibration or jar by an earthquake 
observer, whilst below this number the separate vibrations or shocks can 
be distinguished. This view, I conceive, clears up one very puzzling cir- 
᾿ς eumstance hitherto looked upon as deducible from most earthquake narra- 
_ tives, namely, that there are two distinct sorts of shocks, the explosive and the 
» vibratory, or three, as Humboldt makes out, by adding the vorticose to the 
_ number. It appears just to conclude from all narratives rightly interpreted, 
_ that there is but one order of earth-wave or shock, namely, the normal 
_ wave, and possibly small transversal vibrations transmitted along with it, and 
these capable of reflexion, dispersion, change of velocity, &c.; but that 
_ the rate of succession and the individual intensity of each shock vary in- 
g definitely. 
3 _ 13th. The total duration of motion at a given spot varies indefi- 
ey ἀρορεοέρεος κὸν 
nitely, or between limits which have not been ascertained. 
_____It appears to be established that in the greatest earthquakes, the most 
Ε. violent shocks are very few in number, sometimes only one, usually not more 
*. than three or four, and that to these the great mischief is due, so that in a 
few seconds a vast country is laid waste and its cities and towns overturned, 
as in the great Calabrian earthquake of 1783; that these great shocks recur 
a 
