114 Prof, Cayalleri on a New Seismometers 
locality. I must add that usually, if not always, the undulations * 
are several in number, and therefore the increased force which 
the tautochronous pendulum acquires will always cause it to 
make an arc of oscillation visibly larger than the others, so that 
no doubt can arise as to which of the pendulums is that accord- 
ing with the motion of the earth-wave; but strictly, a single 
tautochronous wave would be quite sufficient. For clearness, we 
have here assumed the terrestrial wave to be horizontal; usually, 
if not always, however, it is mixed, and inclined more or less to 
the horizon. However, in this case it is obvious that, if we 
separate the mixed wave into its horizontal and vertical compo- 
nents, the latter being perpendicular to the horizon, can have no 
influence on the are of oscillation of the pendulum, whilst the 
horizontal foree remains alone active. The knowledge of the 
velocity of earth-waves, besides determining the relative inten- 
sity of earthquakes, may lead to most valuable discoveries, and 
explain many phenomena which now excite strong interest. I 
think that, having gained this knowledge by the aid of our seis- 
mometer, we possess the necessary data for calculating the iten- 
sity of the shock or earth-wave. In short, the intensity of the 
wave may be represented by three distinct elements; viz. the 
vertical altitude which the ground attains, the horizontal lengths 
of the waye, and the time occupied by this simultaneous moye- 
ment, On this supposition, the intensity of the wave may be con- 
sidered in the direct ratio of the two first quantities, and in the 
inverse ratio of the time. Now our instrument being capable of 
marking, Ist, the vertical altitude of the wave by the spiral pen- 
dulum; 2nd, the horizontal undulation by the great pendulum ; 
3rd, the time of the wave as marked by one or other of the small 
pendulums, we have all the elements necessary for calculating 
the intensity of the shock. Lastly, it is clear that with these 
three elements we can make all possible theoretical inferences, 
and assign to each of the three its appropriate value in referring 
to the effects of an earthquake, whether on buildings, on plains, 
or on the sea, &c., in all of which one or other of the three men- 
tioned powers will have a greater or less influence; it has been 
proved, for example, that with an equal degree of intensity, the 
vertical shock will do more damage than the horizontal. Thus 
we can note with these instruments— 
1st. The moment at which the earthquake occurs. 
2nd. The direction of the primary shock or earth-wave. 
8rd. The general horizontal direction of the waves, their am- 
plitude, or length. 
Ath. The height of the vertical wave of shock, however 
complex the vertical and horizontal waves acting together 
may be. 
