72 REPORT— 1858. 
width of the seismic band, paroxysmal efforts are occasionally pro- 
pagated to great superficial distances beyond it. 
6th. The sensible width of the seismic band depends upon the energy de- 
veloped, and upon the accidental geologic and topographic conditions 
at each point along its entire length. 
7th. Seismic energy may become sensible at any point of the earth’s sur- 
face, its efforts being, however, greater and more frequent as the 
great volcanic lines of activity are approached. 
8th. The surfaces of minimum or of no known disturbance, are the central 
areas of great oceanic or terr-oceanic basins or saucers, and the 
greater islands existing in shallow seas. 
The fact that certain low-lying river-basins, such as the Mississippi and 
the Ganges, are the seats of earthquake disturbance, does not conflict with 
the last proposition. In these cases, the impulse is propagated into the plain 
from the band of the bounding ridges; and when these are very large 
in relation to the basin, the breadth of the seismic band may overlap its 
whole surface,—as for example in the basin of the Ganges, where the seismic 
banks of the Himalaya and Vindhya mountains cover the whole plain of 
Northern India. 
We have thus extracted all the information that our Catalogue, or indeed 
any further cataloguing of earthquakes, seems capable of giving us; future 
research must take a more distinctly physical character. I therefore proceed 
to some observations upon instrumental seismometry and the construction 
of seismometers, upon which our future progress must much depend. 
Twelve years ago, at the period of the author’s paper (Trans. R. I. Acad. 
vol. xxi. 1846) “ On the Dynamics of Earthquakes,” the construction of seis- 
mometric instruments appeared a comparatively easy matter; there did not 
seem to be much difficulty in producing even a self-registering instrument 
that should give every element of the earth-wave at the surface, whose nor- 
mal velocity of propagation was then assumed to be extremely great, to 
approximate to that theoretically due to the elasticity of solid rocky media, 
and not to vary very materially in direction of propagation during its transit 
from the origin, to any distant point of the earth’s surface. 
It is only at a very recent period that experiments and observations as to 
the actual phenomena, the velocity and direction of shock, &c. have begun to 
show the real difficulties of the subject ; and as these are apparently not very 
generally recognized, I propose pointing some of them out here, prior to 
indicating the limits within which for the present, it appears to me, we must 
be content to restrict our seismometric aims and instruments, and describing 
what form of instrument, and in what localities placed, would appear, with 
our existing knowledge, the best to give us some information—approximate 
only, and incomplete without doubt, but yet such as can be made a safe basis 
for a future higher step with more refined and comprehensive instruments. I 
shall avoid as much as possible (as out of place in this Report) any mathe- 
matical treatment of the subject. The antecedent history of seismometers is 
in brief as follows :— 
All the instruments hitherto devised or set up may be divided into two 
great classes:—1, observational, those whose motions must be observed and 
recorded after each shock ; 2, self-registering, which record their own past 
movements however repeated, and admit of their observation at any subse- 
quent period within certain limits. Each of these classes is again divided 
into two sorts :—a. instruments dependent upon the movements by displace- 
