a 
3 ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA. 87 
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directions in 2, y, and z, or the horizontal components (a and y) of the 
direction of motion and the vertical component z, will be found the most 
valuable, 
These are determinable by one instrument only. By two or more such, 
at separate and moderately distant places, the velocity of propagation or 
transit-rate @ may be found; and by combining the results obtained by both, 
in calculation, each may be made to check and control the other, and for a 
given seismic region (apart from serious perturbations of internal forma- 
tion) we can obtain the point upon the surface, vertically above the origin 
of the wave, and approximate to the depth of the origin itself, or of the 
focus of disturbance, below the earth’s surface. 
One or other, of two distinct seismometric arrangements, may be adopted, 
both dependent upon similar principles,—the second being of a simpler and 
less expensive character, but not susceptible (as a sizgle instrument) of indi- 
cations as accurate as the first, yet, as respects applicability to determi- 
nations of dime (as one of several, set up in a given seismic area), quite as 
exact, > 
I proceed to describe the construction of both, their principles and action, 
The first instrument is exhibited in Pl. XV. figs. 1,2&3,. Fig. lis a 
lateral geometric elevation of the instrument, whose length is placed in the 
direction N. and S., as seen in plan in fig. 2,—a precisely similar instrument 
being placed at right augles of azimuth to it, or with its length E. and W. 
The same letters of reference apply to similar parts in all the figures. Fig.2 
represents both the N. and S. and E. and W. instruments as placed in posi- 
tion, ww being part of the external wooden shell or wall of the seismic ob- 
servatory, which may best be always of wood, or such material, and circular 
in form. 
In figs. 1 and 2, aa is a cast-iron tabular bar, whose upper surface is 
horizontal, and whose long parallel edges are either N. and S. or E. and W. 
It is attached to a rigid cylindrical vertical bar of wrought iron, 6 6, which 
passes freely, but without shake, through bored holes in the top and bottom 
collars of the heavy cast-iron frame ec, which is firmly bolted by its bottom 
flanch to the heavy stone floor of the observatory; or, if the latter can be so 
placed, to the natural solid rock when levelled to form its floor. Beneath 
the frame cc is a pit, pp, for convenience of access to the bottom of the 
instrument, Upon the vertical bar 8, a collar is fixed of wrought iron, , 
between which and the lower bored collar of the frame ee, a spiral spring, 
é, is placed, having its axis coincident with that of the bar d. 
This spring sustains, when at rest, the weight of the bar and table aa, and 
of all resting upon it, and is so adjusted as to resistance, that such forces in 
the vertical direction, as it may be expected the instrument will be exposed 
_ to at any time, shall not be able to compress the spring to such an extent, as 
_ to bring the lower surface of the table aa, into contact with the top part of 
_ the frame ce. A vertical “feather,” let into the bar b, prevents it, or its 
superior attachments, from altering their position with reference to the frame 
sy § P 
_€e, by turning round the vertical axis of the bar 6 in its collar-bearings, 
A small sliding index, not shown in the figure, also moves in a longitudinal 
groove at the opposite side of the bar db, and, being placed in contact with 
_ the top of the frame ec, when the whole is at rest, indicates the extent of any 
_ Yertical depression of the bar 2, and of its load, by compression of the spring 
_@ A buffer collar of vulcanized india-rubber is placed at J, above the iron 
collar , as a precaution against a jar, in case of the sudden removal of part 
of the load on aa by any accident. 
Upon the upper side and centre of the length, of the tabular bar aa, is 
