and Extraordinary Movements of the Sea. 275 
experiencing any agitations,*—that others experienced them 
at periods widely differing from the general rate of progress, 
—some having taken place in 1755 even half an hour before 
the great earthquake,—are circumstances, mutatis mutandis, 
common also in thunder storms. Shocks of earthquakes, 
when they occur in non-voleanic districts, have been consi- 
dered by many as the effects of electrical discharges from 
the atmosphere into the earth,t or from the earth into the at- 
mosphere.t But whether, during earthquakes, the electricity 
usually passes from the air into the earth, or from the earth 
into the air,—or whether it may not sometimes pass between 
two differently electrified portions of the earth, as lightning 
often does between two differently electrified portions of the 
atmosphere, are points which I believe have not yet been as- 
certained by observation. 
On a former occasion§ I explained how an oscillation of 
the sea might be produced by a simple submarine shock or 
vibration, without any explosion, or the displacement of any 
portion of the bed of the sea. These submarine shocks must 
doubtless often happen without any indication of their occur- 
rence, except the subsequent agitations of the sea. And 
shocks are often felt at low levels without being per- 
ceived at higher elevations, as was the case with the 
shock on 30th December 1832, which does not appear to 
have been felt anywhere in Cornwall except Hayle, on a 
spot only a few feet above the sea. Humboldt,|| too, states 
that in Chili, Peru, and Terra Firma, the shocks follow the 
line of the shore, which is the lowest part of the land, and 
extend but little inwards; he also says that “ sometimes in 
the same rock the superior strata form invincible obstacles 
to the propagation of the motion,—thus in the mines of 
Saxony we have seen workmen hasten up affrightened by 
oscillations which where not felt at the surface.” 
But in 1755, on the day of the great earthquake, shocks 
* Mr Milne, Edinburgh Royal Society Transactions, vol. xv. p. 615. 
+t Rees’ Cyclopsedia.—Earthquake. 
t Jameson’s Edinburgh Philosophical Journal, October 1841, p. 309. 
§ Cornwall Geol. Trans., 1843, p. 117. 
|| Personal Narrative, vol. ii. pp. 222, 224, 
