112 Prof. Cavalleri on a New Seismometer. 
moving the ground with the objects upon it in a horizontal 
direction, also elevate or depress it. These waves, which for 
perspicuity we have called mixed, are perhaps in reality the only 
ones which exist. Several authors, especially geologists, treat of 
these waves,—some from one point of view, some from another, 
and many so hastily that it may truly be said we are yet far from 
possessing a theory of their nature. Such at least is the im- 
pression received from the papers of Gay-Lussac, Humboldt, 
Achille Rossi, Savi, Collegno, Pilla, Favre, and more particularly 
of Dr. Young, who compares (I use his own words) “the earth- 
quake of land to the earthquake of the air.” He also compares 
the shock to the striking of a number of balls placed in a right 
line, of which, when the first is struck, the last only is separated. 
We can do no better, therefore, than closely to adhere to obser- 
vation, and patiently record the components of these waves in 
order to draw deductions from them. In fact, having the ver- 
tical altitude of the wave recorded by the spiral, and the hori- 
zontal deviation marked by the pendulum, we possess all the 
necessary data for mixed waves. Acting thus at nght angles 
(leaving aside for the present more subtile considerations), we 
may regard the mixed wave as the diagonal of these two forces, 
so that the mixed wave would be equal to the square root of the 
sum of the squares of the above-named components. Besides, 
we might also get the inclination which the plane of the mixed 
wave makes with the horizon by considering this diagonal as 
radius, and the vertical altitude given by the spiral as the sine, 
the angle of inclination being that corresponding to this sine. 
A good seismometer ought also to mark the time of the duration 
(tempo della durata) of the shock; but although I do not think 
it impossible to form such an arrangement as should mark this 
also, the problem appears so complicated and difficult that I am 
not at present prepared to attempt its solution. I have rather 
sought to discover the time which the earth-wave employs im its 
excursion, or that of the seismometrical oscillation. It is desi- 
rable to learn how many undulations the earth-wave makes in a 
given time, and thence to see if this do not differ in every instance 
of earthquake and in every country, on account of the different 
strata through which the wave is transmitted, and perhaps also 
on account of the diversity of originating causes. Earthquake- 
waves, so far as I can remember in three distinct cases which I 
have present to my mind, appeared very rapid and almost iso- 
chronous. Recalling these shocks, it does not appear to me far 
from the truth to assign about three undulations per second as 
the rate, at least in our Lombardy Plains. The instrument 
which I have constructed with this view will note the duration 
of these undulations, and consequently whether they are different 
