600 ROTATION BY AN EARTHQUAKE. 
height, or been stripped of some of its architectural ornaments. The 
instance here referred to occurs at the entrance to a large square, 
called EZ Paseo de los Agoas, constructed long since as an artificial 
basin within the city, for boat fights and racing. The tips of the 
various ornaments along the wall are much broken, and the two 
obelisks adjoining the entrance have ‘the upper stone on each dis- 
placed, and turned around on its axis about fifteen degrees in a direc- 
tion from north to east. ‘To understand some points in the drawing, 
it should be remarked that the entrance-way is not at right angles 
with the wall, but a little oblique; and moreover, the obelisks are 
not equilateral, neither are they similar, though both are three-sided. 
These rotations by earthquakes have been attributed by some 
authors to an actual rotary movement in the earthquake vibration. 
But it has lately been shown by R. Mallet, that this hypothesis is 
untenable and unnecessary.* As this author states, a simple vibra- 
tion back and forth is all that is required to produce a rotary motion 
in a stone of a column, provided that stone be attached below more 
strongly on one side of the centre than on the opposite. This may 
be proved by trial. Another explanation occurred to the writer when 
viewing the place above described, and it may be deserving of consi- 
deration. It is admitted that in an earthquake vibration there is a 
line in which the action is most intense, and either side of this line 
the force gradually diminishes as we recede from it. Consequently 
any object within the range of the earthquake will feel less force on one 
side than on the opposite, unless it be situated directly in the centre 
of the track; and hence a rotation might result. If among the rotations 
of a single earthquake they are indiscriminately right and left, Mr. 
Mallet’s theory must be the true explanation. But if there is a uni- 
formity in direction, over a large area, the latter would find some 
support. 
* Phil. Mag., xxvill. 537, June, 1846; and Am. Journ. of Science, ii. Ser. ii, 270. 
