upon the Phenomena, of Earthquakes. 339 
which the wave passed was deeper or shallower*. Thus the 
motion of the wave to Kingsale or Mountsbay (through waters 
not deeper in general than 200 fathoms) was slower than that to 
Madeira, (where the waters are much deeper,) in the proportion 
of about three to five; and it was slower than that to Barbadoes, 
(where its course lay through the deepest part of the Atlantic 
ocean) nearly in the proportion of one to three: so likewise the 
motion of it from the Scilly islands to Swansea in Wales (where 
the depth gradually diminishes from about sixty or seventy fa- 
thoms to a very small matter) was still slower than that to King- 
sale, in the proportion of less than one to three: the same thing 
is observable with regard to Plymouth also,where the wave arrived 
about ninety minutes later than at Mountsbay, though the dif- 
ference of their distance from the first source could not, upon any 
supposition, be more than forty or fifty miles. 
Secrion VIII.—100. If we would inquire into the depth, at 
which the cause lies, that occasions any particular earthquake, I 
know of no method of determining it, which does not require ob- 
servations not yet to be had; but ifsuch could he procured, and 
they were made with sufficient accuracy, I think some kind of 
guess might be formed concerning it: for, 
101. First, In those instances, where the vapour discharges 
itself at the mouths of volcanos, (as in the ease of the earthquake 
at Lima,) it might, perhaps, be possible for a careful observer to 
trace the thickness of the several stratat from thence to the place 
where the earthquake took its rise, or at least as far as the shore, 
if it took its rise from under the sea. If this could be once done 
in any one instance, and the velocity of such an earthquake 
nicely determined, we might then guess at the depth of-the cause 
in other earthquakes, where we knew their velocity, by taking 
the depths{ proportional to those velocities, which probably 
would answer very nearly. 
102. Secondly, If, in any instance, it should be possible to 
know how much the motion of any earthquake was retarded by 
passing under the ocean, the depth of the ocean being known, 
the depth at which the vapour passed would be known also ; for 
the velocity under the water would be to the velocity, if there 
had been no water, in the subduplicate ratio of the weight in 
* We have an instance to this purpose in the tides, which, in deep waters, 
move with a velocity that would carry them round the whole earth in a sin- 
gle day; but as they get into shallower waters, they are greatly retarded: 
and we are told, that in the river of Amazons, the same tide is found run- 
ning up to the tenth or twelfth day, before it is entirely spent. [See Con- 
damine’s Voyage down the Maranon. ] 
+ This is upon the supposition, that the under strata, in ascending up the 
hills, come to the day in the manner before described. See art. 43, and 
fig. 3. } See the note to art. 63. 
the 
