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The self-registration— 
Ist. Of the time of transit, at a given point of the earth’s 
surface, of an earthquake shock, or earth-wave, noting same 
to a small decimal of a second of time. 
2nd. Of the vertical element, or altitude, of the earth- 
wave, at the moment of its transit, whether the wave be a po- 
sitive or a negative one. 
3rd. Of the horizontal element, or amplitude of the wave, 
at the same moment. 
4th. Of the direction, as to azimuth, of the wave transit. 
The principle adopted, as the means by which the wave, or 
shock, shall act upon the instrument, consists in availing our- 
selves of the oscillation of a column of mercury, in two verti- 
eal, and in four horizontal glass tubes, of peculiar construction. 
One end of the column of mercury in each tube is so adjusted 
in contact with one pole of a constant galvanic battery, that 
the oscillation produced in the mercurial column by the wave, 
in passing, breaks contact. The time during which the con- 
tact remains broken is proportionate to the amount of the ver- 
- tical and horizontal elements of the wave. The breach of 
contact releases one or more of six pencils at the instant of its 
occurrence, and until contact is restored. Either of these 
continues to describe a trace upon a ruled sheet, placed upon 
a cylindrical barrel, carried round by the astronomical clock. 
The length of this trace is, therefore, a graphic representa- 
tion of the amount of the respective element of the wave, and 
the pencil which mark& it indicates the direction of the oscil- 
lation, whether vertically positive or negative, or horizon- 
tally from any point of the compass. 
A somewhat similar arrangement marks, upon four dials, 
the hour, minute, second, and fraction of a second, at which 
the crest of the wave has passed the point of the observa- 
tory, or locus of the instrument. This is of peculiar im- 
portance for ascertaining the rate of progress of the wave 
between two distant observatories. The instrument cannot 
