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ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA. 103 
that the depth of the origin (upon that hypothesis) will be always equal to 
the radius of the circle of maximum disturbance. It would be out of place 
here to enter further into the physical discussion of this question, except by 
referring to Herschel (art. “ Light,” ‘Encye. Metrop.’ vol. iv. paragr. 18. 
p- 578) and to the various papers of Cauchy, Wertheim, Stokes, Airy, 
Haughton, and Maxwell on the subject. 
I have stated that in the preceding investigation the effects of the transver- 
sal wave are neglected. In the observation of actual earthquake phenomena, 
this may probably be safely done as respects all points that are at consider- 
able distances from the centre of disturbance. ‘The normal and transversal 
waves, starting at the same instant, appear to travel with unequal velocities. 
They part company ; and their distance becomes greater, and the interval 
larger between their arrivals, the further they have both travelled. Were 
we enabled, therefore, to ascertain the precise velocity of the normal wave, 
and the exact interval of time between the arrival at a distant point of the 
normal and transversal waves, we could still by another method arrive at the 
distance from which they had come, and therefore at the depth of the origin 
of impulse, if the angle of emergence at one point were known. According 
_ to Cauchy, the velocity of transit of the normal is to that of the transversal 
wave as 4/3: 1 in media of unlimited mass; and Wertheim’s modified for- 
mule for elastic bodies fix itas 2:1. My own experimental observations 
with the seismoscope have proved to me that the separation of the two waves 
can be noticed, and the interval of time measured upon even very moderate 
ranges of wave-transit, not exceeding a few miles; and the observations of 
earthquake shocks indicate that one cause of the tremors that usually sacceed 
the main blow, is the later arrival of the normal wave, whose amplitude at 
considerable distances from the origin is always small. 
However this may be, it is certain that in all earthquakes the real mis- 
chief and overthrow, at places pretty far removed from above the centre of 
impulse, are done by the blow from the normal wave, which appears to 
come first; hence the main observable effects are those of the normal, and 
we are justified and enabled, zn such localities, to neglect the transversal. 
But within a considerabie circle of area, whose boundary is evanescent, and 
whose centre lies at the point B (figs. 7, 8), right above the origin, the 
actual effects of the transversal wave are very formidable, and can never be 
neglected. 
_ The ground beneath an object so situated, such as a house or pillar (as 
the distance from the origin to the surface is the minimum range of emer- 
gence, or shortest possible, and therefore its energy the greatest), is almost 
at the same instant thrown nearly vertically upwards by the normal wave, 
and at the same moment rapidly forced forwards and backwards horizontally 
in two directions orthogonal to each other; and this combined movement, 
which is that called “vorticoso” by the Italians and Spanish Mexicans, is 
one that nothing, however solid and substantial in masonry, &c., can long 
_ withstand. 
Hence it follows that, within the zone of maximum disturbance which we 
have treated of, and occupying its central region, we shall always find an 
area, more or less circular, also of great overthrow and destruction, though 
_ presenting entirely different characteristics as to the manner of overthrow of 
the buildings, &c. This middle region may therefore be sought for as a 
further directrix to the point B over the centre of impulse. Jt may be 
necessary to remark that this combined movement, due to the two transver- 
‘sal waves, and dimited to a region closely above the prime vertical passing 
‘through the centre of impulse, must not be confounded by any misconcep- 
