Prof. Cavalleri on a New Scismometer. 105 
column is withdrawn from beneath the small cylinder, which is 
retained in its position by the point of the needle; wanting the 
support, it will immediately fall into the soft ashes, precisely in 
the direction from which the first shock came, that is, from the 
south. This is the first effect of the scismometer, it will indi- 
cate the point from which the first shock came. The shock 
might occasion a very small movement ; yet as we have the power 
of making the point extremely fine, and the cylinder and its sup- 
port extremely small in diameter, it is evident that the slightest 
shocks would affect them, even those of two millimetres or less. 
From the experience of others, and from what has come under 
my own observation, it is certain that the shocks, although very 
weak, would cause a greater deviation than two or three milli- 
metres. It may also be urged that the percussion occasioned 
by the passing of cars, or by thunder, might be capable of 
shaking the walls, and thus giving a fallacious indication of an 
earthquake. To this objection I beg leave to reply, that if the 
walls of the building be firm, and especially if the instrument be 
erected on the ground floor, these extrancous percussions can 
have no sensible influence on the walls, but are limited to the 
air, the window-panes, and such elastic and moveable objects 
as are placed between compressed and compressing air. Although 
passing wheels may communicate a motion to walls to which we 
may apply the word perceptible, it is in reality very slight, and 
almost invisible to the naked eye. For this reason I am led to 
believe that the instrument will not be in the least affected by 
passing cars, and will sufficiently answer our purpose. 
I shall venture to dwell a little longer on this point, and allude 
to everything which may possibly influence our seismometer. 
Shocks of earthquake, however complicated, and as yet not sub- 
mitted to measurement, may be distinguished, as is usually done, 
into undulatory and horizontal, subsultatory and vertical, and 
mixed shocks, the last being by far the most frequent. The appa- 
ratus which I shall attempt to describewill enable us to distinguish 
these three kinds of earth-waves. But there is still a question re- 
lative to the record of the direction of the primary wave by means 
of the small cylinder or prism. When a shock occurs in any given 
point of the globe, or rather when the centre of the earthquake is 
manifested in any given point, we know that undulations proceed 
from that point as from a centre, and are propagated over a cir- 
cumference more or less extensive, according to the intensity of 
the shock or the conducting power of the ground. Now I would 
ask if the first wave, which is gradually extended and enlarged in 
its course, always proceeds from the centre of convulsion to the 
circumference, or whether the reverse can ever happen? Let us 
magine the sudden crushing of a large hollow glass ball from 
