ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA. 8] 
different azimuths, and therein in clearness of indication ; but it also 
has special disadvantages of its own. If, for example, the line of 
earth-wave transit be from S. to N., and the E. and W. pendulum 
be set up at the S. side of its own wall, it will tend to be thrown 
off or out from the wall by the shock; if placed on the N, side of 
its own wall, its friction will be increased on its suspensions, 
and tracing-point, by its being thrown in or pressed against the 
wall; and if the line of earth-wave transit be, say N.W. and S.E., 
both pendula will be either thrown out from or pressed in against 
their respective walls, according to which side of the N. and S. walls 
they be fixed at. This source of variable inaccuracy might perhaps 
be eliminated by a double set of pendula, viz. one at each of the 
opposite sides of the N. and S. and of the E. and W. walls, which 
would thus be oppositely affected (in excess and in defect) by this 
source of error. 
. 9(6). What has been already stated, with reference to errors common to all 
~~ S 
a... 
pendula, and the remarks made under 7 (0) as to the superiority of 
elastic over simple pendula, render it needless to enlarge on those 
which were only proposed as extemporaneous instruments, and for 
which they will be found convenient and useful, and not more in- 
accurate than much more elaborate ones. 
Referring now to the second class, or self-regulating instruments,—the 
disadvantage of the one 
2(a), 
2 (a). 
proposed by the author is of the same character as that of 4 (a) of the 
first class, viz. too delicate a sensitiveness to small tremulous shocks, 
which derange the composure of the instrument, without its giving 
decisive indications. The galvanic recording part of the apparatus 
was all that could be desired, and is of course applicable to other 
forms of instrument as respects the displacement portions. Indeed, 
apparatus identical in all its main characteristics has been since 
brought into successful and constant use by Professor Airy, Astro- 
nomer Royal, for the registration of astronomical and other kindred 
observations, and also by several experimenters abroad. An account 
of many such arrangements will be found in De la Rive’s ‘ Treatise 
on Electricity.’ 
The same remark, I think, may apply to Professor Palmieri’s seis- 
mometer, with this addition: the movement of the mercury, equal 
columns of which are contained in the opposite legs of each U-shaped 
tube, depends in his instrument wholly upon the U-tube heing canted 
over more or less in its own plane, so as to throw the legs of the tube 
out of plumb. This, Professor Palmieri (if I do not misunderstand him) 
considers an inevitable consequence of the transit of the earth-wave 
at the instrument, conceiving the earth’s surface to suffer, in every 
case, such a sensible heaving undulation, as to rock the instrument 
upon it, like a ship upon a heavy ground-swell. I must confess to 
entertaining great doubts that, in the great majority of earthquakes, 
any such sensible undulation (enough, at least, to produce a sensible 
throwing out of plumb of the U-tubes) can occur, although I 
have no reason to doubt that, from its delicate sensitiveness, con- 
tact will be broken, and the instrument act in so far, by some of 
the violent jars or jerks that it may receive. ‘his peculiarity con- 
stitutes, in fact, the essential difference in arrangement between 
the author’s seismometer and Prof. Palmieri’s. In the former the 
1858. G 
