ae 
ON THE FACTS OF EARTHQUAKE PHA NOMENA. 37 
quake Phenomena’), stated that ‘the wave or shock, travelling at the rate 
of perhaps thirty miles per minute, often takes ten or twenty seconds to pass 
a given point; and hence that its amplitude must occasionally be many miles.” 
_ The fact of one shock taking ten or twenty seconds to pass a given point, 
however, is only derived from the narratives of great earthquakes, and from 
the extremely loose use by authors of the word shock, as confounded with 
whole period of motion, possibly consisting of many rapidly successive shocks 
(as already adverted to): this conclusion as to dimension of the wave requires 
to be taken with caution. 
21st. The velocity of transit of the earth-wave or shock has never 
been correctly ascertained for any one locality or occasion. 
A loose approximation was made by Mitchell to the speed of transit of the 
shock in the Lisbon earthquake, from which he deduces a mean velocity of 
about twenty miles a minute, or 1760 feet per second. 
Humboldt states the velocity (‘Cosmos’) to be from five to seven geogra- 
phical (German) miles per minute, which is about twenty or twenty-eight 
statute English miles per minute, and by others various vague and insufficiently 
supported statements of its velocity have been made; but the truth is, the real 
velocity has never yet in any one instance been even approximately ascer- 
tained. No mean velocity, such as those given by Mitchell and Humboldt, 
CAN be true, for if it be granted that the shock isa wave due to the elasticity 
of the materials through which it travels, then the velocity must vary as these 
_ alter, and be dependent on their density and moduli of elasticity. 
_ This we do know, however, that its velocity is extreme in passing through 
_ some formations, and very great in all. ‘“ The ground,” says M. Place, speak- 
ing of the great Chili earthquake, “rose and fell with inconceivable rapidity 
like a mine sprung beneath one’s feet.” Such are his words; and Dolo- 
_ Mieu quotes almost the same as the experience of those who had felt the 
| Calabrian shock at Messina. Thus the shock from below upwards upon a 
British ship at sea, eleven leagues from Manilla, as recorded by De Guignes 
ἴῃ 1796, in his account of the Philippine Islands, was so sharp and sudden 
as to unship and splinter the mainmast; and the Winchelsea, a British ship 
| from Bengal to England, was similarly struck on the 10th of February 1823, 
i in lat. 52° N. and long. 85° 998! E.; and Dr. Percival states, that in the earth- 
᾿ς quake felt in Lancashire in September 1777, “a passage-boat upon the 
Εἴ Bridgewater Canal was stopped in its course as if it had struck upon a cable 
| or other obstacle” (Ann. Reg. vol. xx. p. 79); and ships have been repeat- 
| edly strained so as to leak by such a shock at sea. The velocity of the 
_ shock in sea-water is probably about 4700 feet per second. Stones have 
been observed projected out of walls to a considerable distance by the shock, 
tearing themselves from the mortar-bed; and, what is more direct proof of 
| great velocity, bodies of great stiffness and small inertia have been bent or 
| _ twisted, as for instance, an iron cross and a rod bearing the arms of Hun- 
_ gary, which were both bent by an earthquake at Pesth in the last century, 
_ but my authority for which I have been unable to recover. A somewhat 
_ similar case is recorded by Professor Ferrara (Silliman’s Journal for 1826). 
On the 5th of March 1823,” he says, “ the vane on the top of the palace- 
_ gate at Catania, upon which he bent his eyes, was bowed in a direction from 
ΝΕ, to S.W., and remained so bent 20° from the plumb-line until it fell.” 
* A tall slender palm-tree he saw do the same.” 
Few better proofs can be found of the amazing force and velocity of the 
lateral shock than the overthrow of the Rhodian Colossus—a bronze figure, 
steadied by being filled with stone as to its lower limbs, and cramped with 
lead into the solid masonry of the mole. “ Ante omnes autem in admira- 
A afi 
