74 REPORT—1858, .e “lO 
the broad dimension being transverse to the are of vibration, than 
eitherroundor square as proposed), W the Fig. 2. 
bob, 7 the ratchet and pall. If we suppose 
this to be in the N. and S. vertical plane, 
a shock from the S. may bring the pen- 
dulum into the position p m, when the 
pall will fall into that x, and detain the 
instrument in its new position until the 
angle 2 p W can be observed. 
The main object proposed by the author 
of this modification of the inverted pen- 
dulum was, that the observable movement 
of the instrument should be as nearly as 
possible that of the horizontal component 
of shock, without being perplexed with 
indications due to subsequent abnormal 
motions of the instrument. 
8 (6). The pendulum seismometer of Santi. 
Two pendula suspended close to the faces 
of two walls, ranging in vertical planes 
traversing through the cardinal points, 
are free to oscillate in those planes only. 
Each is provided with a chalk tracer, which marks the are of oscil- 
lation N. and S. or E. and W., or vice versd as to either, upon the pre- 
pared face of the wall. This has been long in use in Italy. The length 
of the horizontal chord of the are traced is assumed to be equal to 
the horizontal component of shock in the direction marked, and inter- 
mediate movements are to be obtained from comparison of the lengths 
of both cardinal chords by the known laws of compounded motions. 
9 (6). A vertical inverted spring pendulum, formed of an elastic rod (wood 
or cane), with bobs of iron shot, is fixed within a hoop, with certain 
extemporaneous means of marking its oscillations in any plane, or 
more than one, for horizontal component. Such pendula, fixed hori- 
zontally in a wall, or in two N. and 8. and E. and W. walls, may be 
used for vertical element, or a shot hung from a spiral spring of wire 
(Mallet, Admiralty Manual, sect. vii. p. 217, 218.); these were in- 
tended for extemporaneous use. The spiral spring arrangement has 
had several different proposers, some anterior to the above. 
Such are the principal instruments of the first class, used or proposed, 
in addition to which may be noticed the balanced circular dish, or wheel- 
formed seismometer, suggested, I believe, by Professor J. Forbes and Col. 
James, R.E.,—a disk of cast-iron or other metal with a heavy rim, upon a 
central point of suspension slightly above the centre of gravity, and provided 
with a central tracing-stile, either above or below. The sensibility and power 
of horizontal recovery or stability of this instrument are nearly identical 
with those of the common balance. It is liable to all the objections that 
apply to pendula, whose properties in oscillation it still partakes of; and it 
is difficult to see any one special advantage offered by it. 
Of the second class, or self-registering seismometers, the number is much 
more limited. 
1 (a). The first completely self-registering seismometer proposed, the author 
believes to have been that invented by himself, an account of which 
