Mr. Mauuet on the Dynamics of Earthquakes. 5T 
same time, thus converting the rectilinear into a curvilinear motion in space. 
The relative amount of the two compounded motions being dependent upon the 
velocity and time of movement of the base, and upon the perpendicular distance 
measured horizontally at the surface of adherence between the centre of adhe- 
rence and the centre of gravity of the body. 
This latter case is that which appears to have twisted the stones of Calabria’ 
South America, and Greece, and affords, as I feel assured, the true explanation 
of the phenomenon. 
The relations of these forces, which have taken so many words to state cor- 
rectly, might, of course, have been expressed algebraically in three lines, but as 
this would not be universally intelligible, I have preferred the more tedious and 
inelegant statement of words, and, to render the matter quite familiar, have pre- 
pared a model of one of the Calabrian pedestals figured by Mr. Lyell, which will 
exhibit to the eye all the phenomena already adverted to, on communicating by 
the hand a rectilinear horizontal motion to the base.* 
I have now proved that no vorticose motion is requisite to account for the 
twisting of bodies, as observed in earthquakes; that nothing more than a simple 
horizontal rectilinear motion is demanded. But it may be asked, if this motion 
in earthquakes be also an alternate one—if the earth shake both back and for- 
wards—how is it that these and other displaced bodies are not replaced by the 
reverse motion—by the same sort of motion acting in the contrary direction ? 
This question is, I believe, fertile in consequences, and its consideration has 
led me to some further conclusions as to the nature of earthquake motion. After 
looking through a great number of authors on earthquakes I have not been able 
to find one that has endeavoured, far less succeeded, in shaping to himself any 
distinct notion as to what the precise nature of the earthquake movement is. The 
ancients, appealing to their senses, so far as these could guide them, thought 
that it was like the shaking of a sieve, as the word aecpos tells us. The mo- 
derns, in general, are not more exact in their notions; ‘a trembling”’—‘“ a vibra- 
tion’’—“‘ a concussion” —“ a movement,”’ and soforth, are the words we find scat- 
tered through even scientific authors: Michell, Lyell, Darwin, with some others, 
although they obviously have formed no distinct idea on the subject, use the 
*« The model was exhibited to the Academy. 
VOL. XXI. I 
