ON THE FACTS OF EARTHQUAKE PHZ NOMENA. 33 
τ΄ ease, it has been stated that one shock is observed moving nearly horizon- 
ξ tally, and the other nearly vertically, or very much inclined to the horizon. 
___ f& There is no good evidence of two shocks arriving together, or nearly so, 
at one point, both horizontally, and from different points of the horizon. 
Such an occurrence has been often inferred from phenomena admitting 
of another solution (as we shall see when speaking of vorticose move- 
ments); but there is no ὦ priort reason why, from two distant centres 
of impulse, horizontal shocks should not be felt at once; and this seems 
to have been remarked by Humboldt in Asia. 
18th. The motion of translation of the earth-wave or shock is 
rectilinear and not curvilinear. 
_ All observers concur in stating, that the general direction of motion of the 
shock, whether horizontal, inclined or vertical, is rectilinear. Such is the 
testimony of the unaided senses. It is also the conclusion to be drawn from 
the motions observed as given by the shock when more or less horizontal, 
to fluids in bowls or tubs, to pendulums, to candelabra in churches, to 
furniture, &c.; and from the motions directly given to men, who have felt 
_ themselves “jerked upwards,” as in New Zealand (West. Review, July 1849); 
_ or to articles thrown on to others, asa barrel standing close to a mass of jars, 
and caused to leap up upon them, as at New Zealand ; or to a ship’s mast, un- 
_ shipped by a shock from beneath at sea. 
But from a misinterpretation of some such phenomena, authors of the 
highest ability have affirmed, that there are other shocks which are not recti- 
~ linear in their motion of translation, but consist in avorticose or twisting motion 
of the ground, a rapid rotation in fact of any given point round some distant 
centre. The Italians distinguish popularly three sorts of shocks, the orizontale, 
_ the oscillatorio and the vorticoso. The twisting of the Calabrian obelisks has 
been given as the most convincing proof of this; and Humboldt (Cosmos) says, 
* Circular or rotatory concussions are the rarest, but they are the most dan- 
 gerous of all. The twisting round of the steeple of the church at Inverness, 
_ Seotland, on the 13th of August, in the year 1816, as related in Tilloch’s 
᾿ς Magazine, vol. xlviii. p. 150, though a little known or noticed instance, is a far 
‘more remarkable one than that of the oft-recited obelisks. Twisting round of 
walls without throwing them down, plantations of trees which had previously 
stood in parallel rows deflected, the directions of the ridges of fields covered 
with grain altered, were observed at Riobamba, Feb. 4, 1797, and in Calabria, 
Feb. 5and March 28, 1783.” He adds, “ With the latter phenomenon of rota- 
tion, or the transposition of fields and cultivated plots of ground, of which one 
‘has occasionally taken the place of another, there is connected a translatory 
Motion, or mutual penetration of several strata. When taking the plan of the 
 tuined city of Riobamba, I was shown a place where the whole furniture of one 
_ dwelling-house had been found under the ruins of another. The loose earth 
of the surface had run in streams like a fluid, of which it must be conceived 
that it was first directed downwards, then horizontally, and finally upwards.” 
I maintain that there is no evidence whatever, from any observed facts, 
for assuming any vorticose motion of the shock, or any other than a recti- 
— one. The case of the Calabrian obelisks and of the church of La 
_ Merceda at Valparaiso, I believe it is admitted that I have disposed of in my 
aper on the Dynamics of Earthquakes, read to the Royal Irish Academy in 
1846, and shown that rectilinear motion is sufficient to account for all 
Such cases of twisting. In the cases above adduced by Humboldt, he has 
- fallen into the greater error of mistaking the secondary effects of landslips, 
and THEIR twistings of the land, for those of vorticose motion, as I shall more 
ἢ σαν explain when treating of the secondary effects of earthquakes. 
ν᾿ 0. ‘ D 
ΒΗ 
