and extraordinary Agitations of the Sea. 47 



ting currents occupy generally, in each efflux, as well as in each 

 influx, from five to ten minutes. 



Borlase, in his ' Natural History of Cornwall/ as well as in 

 the Transactions of the Royal Society, has described similar dis- 

 turbances in Mountsbay during the last century. And I have 

 in the f Transactions ' of this Society given descriptions of several 

 in the present century, which are now recorded also in my work 

 'On the Land's end District/ published in 1862. 



To explain these phenomena when accompanied by known 

 earthquakes, Mr. Mallet has written an article in the ' Quarterly 

 Journal of Science' for January 1864 (No. I. p. 68), into which 

 journal was merged that year the ' Edinburgh New Philosophical 

 Journal/ which contained my numerous papers on the subject, 

 read before the Society during the years 1843-60. The follow- 

 ing is his hypothesis : — 



" Should the origin of the earthquake be under the sea, then 

 at the point passed through by the seismic vertical, and around it, 

 the sea-bottom is, as on land, suddenly upheaved and again 

 dropped down ; or it may be, as by a submarine volcano, actually 

 broken up altogether, and steam, lava, and floods of lapilli, &c. 

 may then be belched out under water. In either case there is 

 forced up a volume of water upon the sea's surface just above, 

 or several of these in succession ; and as each mass falls again it 

 assumes the horizontal form of a circular liquid wave of transla- 

 tion ; and these are propagated outwards over the surface of the 

 sea, like the circles or ring-shaped waves on a pond when a 

 pebble is dropped into it. The altitude and breadth of these 

 waves depend mainly upon the magnitude of the disturbance at 

 the bottom, and on the depth of water above it : the rate of their 

 propagation outwards has nothing to do directly with elasticity ; 

 it is dependent simply upon the square root of the depth of the 

 water traversed by the wave on its surface. * * * * When the 

 long flat swell of such waves as they are originated in the deep 

 sea approaches the shores and reaches shoal waters, their fronts 

 become sharper and steeper, and they finally roll in upon the 

 shore as the great sea-waves of South American and other earth- 

 quakes so much dreaded. * * * The great sea-wave of transla- 

 tion rolls in, often hours after the shock has done its work of 

 destruction ; or portions of it may roll in upon shores that have 

 felt no shock at all. Thus in the great earthquake at Japan 

 which a few years ago wrecked a Russian frigate in one of the 

 harbours there, the great sea- wave produced in the deep sea 

 near those islands, hours afterwards reached the opposite shores 

 of the Pacific, at St. Diego and Francisco." 



This hypothesis, although advanced by the most eminent 

 English writer on earthquake phenomena, appears to me highly 



