Prof. Cavalleri on a New Seismometer. 113 
in various countries and in different earthquakes. The principle 
upon which the instrument is constructed is very simple. 
Let us imagine a pendulum formed by a ball suspended by a 
wire, and this wire attached to a moveable point of suspension. 
If this point of suspension be moved forward and backward a 
certain distance in the same horizontal line, moving to and fro 
in equal times with the time of oscillation of the pendulum itself, 
then the are which the pendulum makes continually increases 
with the increase of the motion to and fro of the point of sus- 
pension. If, however, the motion of this poimt occur in times 
which are not isochronous with the oscillations of the pendulum 
itself, its are of oscillation will become less. In the first case, to 
the momentum which the pendulum acquires from being dis- 
placed by following the altered perpendicular of the point of 
suspension, must be added the momentum communicated by its 
moving to and fro, and so the arc of oscillation is increased. In 
the other case the force of the moving to and fro is partly or 
entirely subtracted from the vibrations of the pendulum, since it 
acts more or less in a contrary direction ; hence the are of vibra- 
tion is diminished. A reciprocating motion tautochronous with 
the pendulum causes the greatest arc of oscillation. These con- 
siderations being premised, I take a strong bar inclined some 
degrees towards the horizon. To this bar, and at such distances 
as shall exceed the amplitude of the largest earth-wave occurring, 
I fasten a number of small pendulums, as represented in the 
Plate. These pendulums terminate in sharp needles which touch 
the ashes beneath them, so that when a shock occurs they leave 
impressed the traces of their vibrations. The ashes are hollowed 
out like a cup, in order to assimilate to the ares traced by the 
various pendulums, and prevent the deep impression of the 
needles, which might partly hinder their vibrations. The lower 
extremities of the pendulums form a horizontal line, as in the 
figure, so that their lengths vary: they are ten in number, 
which I consider sufficient. I have arranged them in such a 
manner that the shortest will make a little more than four oscil- 
lations per second, and the longest two. These two limits, of 
about four and two oscillations per second, appear to me suffi- 
cient to embrace every undulation occasioned by any earthquake. 
Let us suppose the pendulum to be set in motion by a shock. 
All the pendulums will vibrate, and leave separately traces of 
their oscillations impressed on the ashes; and the pendulum 
which has marked the largest are will have performed its oscilla- 
tions in equal times with those of the earth-wave ; so that by find- 
ing the square root of the length of the pendulum, or, better, by 
actually observing its time of oscillation, we shall learn the dura- 
tion of the undulations of that particular shock and in that 
Phil. Mag. 8. 4. Vol. 19. No. 125, Feb, 1860. I 
