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: ON THE FACTS OF EARTHQUAKE PHENOMENA. 35 
transversal waves, of much less dimensions, and whose time of arrival at a 
given distant point from the origin, will be somewhat later than that of the 
normal wave; and where the normal is transmitted in a direction horizontal, 
or nearly so, these transversal secondary waves will be felt as a short tremor, 
or shaking up and down, and crosswise to the line of translation of the normal 
wave, and almost at the same time with it. (See Poisson, Mem. Acad. Scien., 
1816, 1817, 1823.) 
This combination of motion is clearly described by Aristotle; and when 
experienced by alarmed persons, unused to precise observation, may well 
give rise to the extraordinary and perplexing accounts of the nature of the 
movements which abound in earthquake narratives. 
19th. The earth-wave or shock has in all cases a true wave form 
upon the surface of the earth, and when its direction of trans- 
lation is guam proximé horizontally along the earth’s surface, 
the crest of the wave advances along a given line and parallel 
to itself. 
_ For this more perfect evidence is desirable. In the case where the shock 
comes up vertically, or nearly so, from beneath, we have evidence that the 
demolition of buildings, &c. has been greatest where the shock has been felt 
most vertical, as under the town of Oppido in the great Calabrian earth- 
uake, and that all around this the destruction became less and less as the 
᾿ς direction of the shock was more inclined, yet not diminishing with a very 
_ strict regularity. 
᾿ς Now as in such a case the mere change of direction of shock from vertical 
_ to inclined would have the directly opposite tendency, if taken alone, as the 
; inclined shocks all around would, if equal in extent of movement, throw 
_ down buildings, &c. more effectually than a vertical one (as is evident), the 
change in destructive power must have been due to another cause, namely - 
to the actual amount of motion having been greater at the centre under 
Oppido than in circles receding around it. Hence we conclude that the 
_ greatest amount of motion was at the centre, where the shock was vertical 
‘at Oppido, and that here the crest of the wave was raised the highest ; that, 
in fact, at the moment of the shock the whole surface was momentarily raised 
_ into a very flattened dome-shaped wave (the height of the dome being of 
_ course extremely small as compared with its diameter), and again dropped 
_ down to its former configuration of surface as the wave passed outwards at 
all sides. 
__ Whatever differences may be due in such cases to differences in terrene 
formation, this must never be overlooked, namely, that supposing a shock 
_ transmitted through a perfectly homogeneous mass from a deep centre of 
effort, and the pulses passing outwards in all directions in spherical shells, 
__ there will be a circle, upon the earth’s surface, somewhere at a determinate 
~ horizontal distance from the central point vertically over the centre of effort, 
in which the upsetting or overturning power of a shock of given intensity 
- will be greater than at any point within or without this circle; within, be- 
ἘΠ 
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