1874.] Alternating Currents. 159 
horizontal portion of the basin of a lake occasions no 
current, because the surface of the water is thereby jerked 
up in jets perpendicular to the horizon, which jets fall back 
into the places they had previously occupied. An illustration 
of this appears in the following extract from the ‘‘Times ” of 
October 18, 1869, describing the effe¢éts of a vertical shock 
from a horizontal portion of the bed of the sea, over which 
a ship was then passing :—‘‘A correspondent writing from 
Valparaiso, on 3rd September, 1869, says, ‘I arrived here on 
the 28th ult, in the steam ship “‘ Payta,” from Callao. 
We left Arica in the morning of the 24th ult., and at 1.20 p.m. 
(the ‘“‘Payta’”’ being two to two anda half miles from the 
land, fifty seven miles south of Arica, off Gorda Point, and in 
seventy-five fathoms of water), the vessel commenced to 
shake in the most awful manner, making it impossible to keep 
one’s footing. (In another account in the same day’s paper 
by the purser of the ‘‘ Payta,” it is stated that ‘the vibration 
threw the passengers to the height of two inches* above the 
deck’). This convulsed vibration continued 45 or 50 seconds, 
during which time the sea presented the appearance of a 
heavy fall of large hailstones, sending up jets of water some 
eight or ten inches, and being about the same distance apart. 
On arriving at Iquique the same night, we learnt that two 
very severe shocks of earthquakes had been felt there about 
two o’clock in the day.”” Had this vertical shock off Gorda 
Point proceeded from an inclined shore, the jets of water, 
instead of being jerked up vertically, and falling back into 
their previous places as they did, would have been jerked up 
obliquely in a seaward direction, rising so little above the 
surface that the separate jets would not be distinguishable, 
and the entire surface would have been seen as a vast 
current rushing seaward, so long as the vibrations continued ; 
and, when they ceased, the heaped up water would return 
shoreward to find its level. 
We have seen that in Lake Ontario, in 1845, as well as in 
1872, the water first retired to a depth of two feet or more, 
and then returned to an equal height above its previous 
level, just as a harp-string or pendulum, when moved to any 
distance from its point of rest, will immediately of itself move 
twice such distance in the opposite direction. This applies to 
marine as well as to lacustrine disturbances. Luke Howard, 
describing those at Plymouth on the 11th of May, 1811, 
says: ‘“‘the sea fell instantaneously about four feet, and 
* Lyell records an instance of men on board a ship forty leagues west of 
Cape St. Vincent being thrown by an earthquake shock ‘a foot and a half 
perpendicularly up from the deck.”’ 
