106 Prof. Cavalleri on a New Seismometer. 
which the air has been exhausted; is it not evident that, at the 
moment of rupture, the surrounding air will rush violently in to fill 
the vacuum, and the first wave will come from the circumference 
to the centre; and that the same may be affirmed of the other more 
distant waves which successively enter the ball? On the other 
hand, should a certain quantity of gunpowder be ignited, in con- 
sequence of which a gas is formed which demands a new or 
greater space, will not the first wave proceed from the centre to 
the circumference? To make use of a better expression, we may 
call this last wave positive, and the first negative. This ques- 
tion might be theoretically treated by mathematicians, and their 
considerations might be of essential service in throwing light on 
the still obscure origin of earthquakes. 
An earthquake may be produced from two very different causes 
with reference to our mode of considering the wave. It may 
happen that a considerable quantity of water or other matter 
may instantaneously produce such a volume of gas or vapour as 
will raise or displace in some manner a portion of the terrestrial 
crust and afterwards allow it to return to its primitive position ; 
on this supposition the first wave must be positive. But if the 
steam or gas be slowly formed and expanded gradually (with in- 
creasing tension) till it finds instantaneous vent in the open cre- 
vices of the earth’s crust, the first wave must be negative. In 
either case, laying aside the theoretical consideration of the ever 
difficult problem of the waves, of which we know neither the 
origin nor the depth, nor the medium through which they are 
propagated, nor the great and various pressure of the different 
strata, our pendulum may solve the practical question for us ; 
for should we find, after a certain number of earthquakes of 
which the centre of effort has been subsequently ascertained, 
that our little cylinder had been displaced either in the direction 
of that centre or in the opposite direction, we might infer whether 
the earthquake had originated from the first or from the second 
of the supposed causes. Besides, we may conclude that the ne- 
gative wave must rapidly decrease in strength as it is removed 
from the centre, and cannot be sensibly felt as far as the posi- 
tive wave, which is in its nature much more powerful. Some 
experiments, although very imperfect, which were tried on the 
surface of a lake on which were placed wood floats, bearing card 
cylinders balanced so as to fall easily at the agitation of the water 
either in a positive or a negative sense, led me to conclude that 
the negative wave must be extremely weak. The accuracy of 
our instrument in noting the direction of the primary wave may 
throw greater light on another phenomenon, and perhaps com- 
pletely solve it: I mean the phenomenon already noticed by 
some geologists, that the earth-waves produced by shocks take 
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