γ ΝΣ 
Ν b 
Ρ 
ON THE FACTS OF EARTHQUAKE PHZNOMENA. 45 
gulated force as is sufficient to affect the air, from some quarter near the 
- hearer, most probably by the vibration of great tabular or mural surfaces of 
rock, but insufficient to shake the ground under his feet; or sometimes to the 
impulse of shock, being principally directed by circumstances of formation 
in one direction, while only enough of the original impulse is enabled to 
pass in others towards the hearer to affect his sense of sound, without his 
feeling a shock. Where the resonant surfaces are so vast as in these cases, 
extending over a whole surface of rocky country, a very slight vibration 
will produce an overwhelming sound, of which some familiar illustrations 
have been already given. 
In every case it should be borne in mind, that from a distant horizontal 
centre of impulse, or rather from a point vertically above it, two sets of sound- 
waves must arrive to the hearer from each shock capable of being heard at 
all, viz. one coming through the earth rapidly and directly, and the other 
emerging first vertically upward through the earth to the surface immediately 
above the centre of impulse, and transferred thence laterally through the air 
at the usual rate; hence any single blow delivered in the depth of the earth’s 
erust will be heard, if heard at all, at a distance, not as a single, but as a pro- 
_ longed rumbling sound, or as two distinct sounds. 
- We now pass on to the consideration of the effects of earthquakes upon the 
ocean or sea when their centre of impulse is beneath it. 
24th. Where the centre of impulse of an earthquake is under the 
sea, and within a certain distance (usually a comparatively small 
. one) of the land, the sea at about the moment that the shock is 
ἦ felt by an observer on the shore is seen to swell and to retire 
ξ from the beach slightly, and at a certain interval of time after 
q the shock (dependent upon the distance of the centre of im- 
. pulse), a great sea-wave of translation rolls in upon the shore. 
The originating impulse of earthquakes being either 1st under the sea, or 
_ 2nd on dry land, gives rise to some difference in the nature and succession 
: of the phenomena constituting the whole phase of one complete shock. Thus 
| considered, ἃ priori, if the origin be inland, we may, if stationed on the beach, 
| have the following succession of waves for one complete phase of shock :— 
i 
| 1. The earth-wave of shock, accompanied and perhaps preceded and fol- 
lowed by sound-waves through the earth. 
__ 2. The sound-waves through the air. 
8. The forced sea-wave, as I have denominated the small wave produced 
on the beach at the moment that the earth-wave of shock either plunges 
| ἢ _ beneath the sea, or vice versd, emerges from it when the origin is under 
____ the ocean. 
If the origin be under the ocean, then the succession for one complete’ 
phase of shock will be— 
__ 1. The earth-waves of shock and sound together, or nearly so. 
2. The forced sea-wave lost upon the beach. 
__ 8. The sound-wave through the sea. 
᾿ς 4 Sound waves (possibly) through the air. 
__ δ. The great sea-wave of translation, which rolling in upon shore with 
Εἶν immense velocity and violence, completes the catastrophe. 
__ Some of these may be wanting in given instances, or the order of the 
~ sound-waves may be slightly different from causes already adverted to, but’ 
ἢ € above-named order of theoretic succession represents nearly that which 
_ has been recorded of most great earthquakes. 
| _ Inthe records of many of these, the peculiar wave phenomenon occurring» 
