270 Mr D. Milne on Earthquake- Bhocks in Great Britain^ 



of maximum violence, Lave been great, is evident from tliei 

 distance to which the wave was propagated, and its consider- 

 able height. But such an elevation of the level of the sea, in 

 any one place, would have the effect of drawing towards it the 

 water in adjoining parts, and thus lowering their level ; and 

 for the same reason, the advance of the wave would everywhere 

 be heralded by a depression of the waters. 



Whether the wave is produced by a convulsive vertical heave 

 of the bottom of the sea, or by the mere effects of a vibration 

 transmitted through the earth's crust, is a question which ad- 

 mits of some doubt. In the Lisbon earthquake, there were 

 appearances which support the former supposition ; for, on 

 the night preceding the earthquake, before any shocks were 

 felt, the springs at Colares and Lisbon were strongly affected, 

 affording proof that the general mass of the earth's crust had 

 begun to be acted on from below ; and it was ascertained that 

 it had been raised by the earthquake in different places, both 

 within and beyond the limits of the ocean.* 



It is by no means intended to be affirmed, that mall earth- 

 quakes there is, in addition to a vibration upwards, an up- 

 heaval of the earth's surface en masse. Such, however, ap- 

 pears to have been the case in the Lisbon earthquake, and it 

 is probably the cause of those great elevations of the sea which 

 frequently accompany earthquakes. Such extraordinary fluxes 

 and refluxes are not uncommon in the harbours of our own sea- 

 girt country ; and are probably to be ascribed to earthquakes 

 in the ocean, of which no other indication reaches us. 



The foregoing remarks are much more than sufficient to 

 illustrate the stature of the shocks^ and particularly the tremu- 

 lous motion and the concussion, of which the shocks most fre- 

 quently consist. Some other points have, in the course of 



• In the account given in the Pliilosopliical Transactions for 1755, (page 

 41G), of the phenomena at Colares (20 miles north of Lisbon), it is men- 

 tioned, '' that in a passage between certain rocks and the main land, vessels 

 had previously sailed, even at low-water, and now you may go to them at 

 low-water without wetting your feet." It is added, that " in other places 

 it appears, by the cliange of the currents, that the earth was moved, so that 

 some spots are more elevated, others more depressed, than before." 



