26T 



The self-registration — 



1st. 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- 

 cal, 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 marKS 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 



