96 Mr. Ma.tet on the Dynamics of Earthquakes. 
fact, in waves that have originated altogether below the ken of our upper 
world? 
While the facility with which one class of our data may be ascertained will 
be disputed by none, it may, perhaps, occur to some that, as earthquakes are 
happily rare, and give no notice of their advent, and moreover, are times of such 
consternation, so but little accuracy is to be hoped for in observations, as to the 
speed or circumstances of the shock, made during such visitations. This might be 
partly true, were we dependent upon the nerve or watchfulness of individual ob- 
servers; but already attention has been given to the contrivance of self-acting instru- 
ments (and instruments, though by no means well devised nor self-registering, have 
been already in use in Scotland, and perhaps elsewhere) for the registration of earth- 
quake shocks; and there can be no doubt that, by earthquake observatories, estab- 
lished, with suitable instruments, at distant localities, in South America or Central 
Asia for instance, where earthquakes, greater or less, are of almost daily occurrence, 
a very complete knowledge of the time of wave transit, and of the amplitude and 
altitude of the earth waves for given districts, would be soon obtained. No instru- 
ments for ascertaining the latter have been yet proposed, but they do not seem 
by any means difficult to devise. It is almost certain that minute earthquake 
shocks frequently pass through almost every part of the earth’s surface, so slight 
as to remain unnoticed for want of instruments to detect them. In February, 1822, 
a slight shock of an earthquake was felt at Lyons, in the south of France ; it was 
not perceptible generally at Paris at all, yet the wave reached and passed beneath 
that city ; for M. Arago, who was engaged at half past 8 o’clock in the morning in 
his magnetic observatory, found that his large magnetic declination needle, which 
was previously quite at rest, suddenly began to swing like a pendulum in the di- 
rection of its length, viz. north and south; and M. Biot, who was at his residence 
in the College de France, observed other analogous phenomena there.* It 
would, therefore, seem very desirable that suitable instruments for earthquake 
registration were, at least, added to all the magnetic observatories now so widely 
extended over the earth, accompanied by proper instructions to the observers,— 
unless, indeed, separate geological observatories be established in favourable 
localities for taking cognizance of all movements of the earth’s crust. 
* See Comptes Rendis.—Prof. Lloyd has also, at a subsequent period, very frequently noticed 
such phenomena at the Magnetic Observatory of the University of Dublin. 
