Mr. Matter on Earthquake Registration Instruments. 111 
there until the restoration of the current enables the magnet again instantly to 
lift it off; and in the interval, as the cylinder has continued to revolve, the pencil 
has marked a line, the length of which is in proportion to the interval of time 
during which the current was suspended. 
When an earthquake positive wave passes the instrument, a sudden jerk 
upwards is given to the whole instrument; the column of mercury rs is therefore 
momentarily depressed by its inertia, and the upper surface 7, receding from the 
extremity of the wire ¢, contact is broken, and the current ceasing to pass round 
the bar g, it ceases to be magnetic, and the pencil instantly commences to mark 
upon the ruled paper on the cylinder, until, by the restoration of equilibrium in 
the mercurial column, after the transit of the wave, the current is again restored. 
Now, as the depression of the surface of the mercury 7 is proportionate to the 
altitude of the positive wave, and as the length of the mark made by the pencil 
is proportionate to the same, the pencil registers the altitude of the wave. 
It is possible that negative earthquake waves of horizontal transit occasionally 
pass the earth’s surface (those in which the wave crest is a hollow, instead of an 
elevation). Toregister these the syphon tube gis arranged similar in all respects 
to p, except that the arrangement of the wires with reference to the bulbs is 
reversed—the wire in the lower bulb s being just in contact, and that in the upper 
bulb 7, cmmersed in the mercury, so that a sudden jerk downwards, may break 
contact, in place of one wpwards, as in the former case. 
The registrations of p are made with a black pencil, /, on one side of the 
cylinder, and those of g with a red pencil, /’, upon the opposite side, both being 
registered upon the column BAC of the ruled sheet ; two out of the six marking 
apparatus are thus employed; the remaining four apply to the syphon tubes, 
a, B, y, & which register the horizontal element of the wave, and its direction in 
azimuth. These four tubes, only three of which are visible in the figure, are 
placed with their horizontal limbs, 6 and @ north and south; a and y east and 
west. 
The arrangements of the surfaces of the mercurial columns, and of the wires 
and magnets, are precisely similar to those described for the vertical elements, 
except that contact is broken in each case at the surface in the horizontal limb. 
Thus, when the wave passes from south to north, contact is broken in the tube ; 
when from north to south, in the tube é ; and so by east and west shocks. When 
