1874.] Alternating Currents. 157 
@mtario on the 13th of June, 1872, as related in the 
“Rochester Democrat ” of the 15th of that month. While 
some gentlemen of Rochester were in a boat near the beach, 
** where the water is usually 2 feet deep at least, their boat 
suddenly grounded, and the waters receding left her on a 
sand-bank. The gentlemen got out and strolled away, but 
looking back shortly after, they saw to their surprise the 
boat dashing about in apparently deep water. Securing the 
boat with some difficulty, they found her suddenly aground 
again, and as suddenly floated after a short interval. 
Becoming now interested in this curious ebb and flow of 
the lake, they diligently observed it for about three hours. 
The ebb and flow occurred every twenty minutes, that is, for ten 
minutes the water would gradually recede, then commence rising, 
and continue to rise for about ten minutes. The water rose 
2 feet and 3 or 4 inches above the ordinary level, then receded 
about the same distance below the usual level, making a variation 
_m the height of the water of nearly or quite 44 feet every twenty 
minutes.’ The above quotation is from the ‘‘ Croydon 
Chronicle” of July 13, 1872, and what I have italicised is 
also a quotation from an abbreviated notice in the ‘‘ Times” 
of the same date. 
There was a similar occurrence in this lake on the 2oth 
of September, 1845, when the waters suddenly moved “in 
a mass out of the rivers, bays, coves, harbours, &c.,” to 
a depth of 2 feet, and then returned to an equal height 
above their previous level. This happened on each side of 
the lake.* 
The explanation of these phenomena, whether in lakes or 
on the shores of the sea, I have given in the “‘ Transactions 
of the Royal Geological Society of Cornwall” for 1843, and 
as it is the only one yet proposed that can reconcile all the 
observed facts connected with them, I will now give it more 
fully, and with the addition of some recent confirmatory facts. 
My hypothesis is that an earthquake shock proceeding 
upwards vertically from the interior of the earth reaches the 
basin of the lake. Now wherever this basin is horizontal 
the shock continues its vertical ascent up through the water 
as through a solid body, and with greater velocity than the 
most rapid flight of a cannon-ball; but none of the water is 
thereby displaced except the surface, which, as will be pre- 
sently exemplified, is dashed up vertically and then falls 
back into its previous place, no current being produced. If 
a ship were on the spot, it would receive the shock, and 
* Edinburgh New Phil. Journal for July, 1848, pp. 107-109. 
VOL. V. (N.S.) x 
