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placement.)
85
if defined as the maximum acceleration of the ground wave,
intensity could be calculated from the maximum period and displacement
of the largest wave, quantities that could be directly recorded by existing
instruments. But Holden never paused to justify his claim that the
maximum
acceleration
really expressed the
intensity
of a shock.
86
Seismological practice
suggested otherwise. Mallet, for instance, had assumed that intensity could
be measured by the distance an object was thrown by a shock: that was pro-
portional to the
maximum velocity
of the ground.
87
Seismology's challenge, as the American engineer T. C. Mendenhall soon
pointed out, was to clarify the term “intensity.” Mendenhall observed that
his colleagues tended to confuse two fundamentally different meanings of
the term. One was the “destructiveness” or “destructivity” of the seismic
wave: this measured the earthquake's “power to destroy” and was appro-
priately associated with the maximum acceleration of the ground, as in
Holden's definition of intensity. The other meaning was Dutton's: “the mag-
nitude of the [earthquake's] subterranean cause,” or “the energy involved
in an earthquake.” if the seismic wave was assumed to be approximately
harmonic, like a vibrating string, one could apply to it the mathematics of
nineteenth-century acoustics. True, the amplitude and period of the seis-
mic wave at the surface varied greatly from one point to another, but this
was likely the result of variations of the elasticity of the ground, not of the
subterranean wave. On this acoustic model, seismic intensity would be de-
fined in terms of the time-averaged rate at which the wave transfers energy
through a region of space. This, in Mendenhall's judgment, was “the most
important sense” of intensity in seismology, and a meaning far removed
from the “destructivity” of an earthquake. Mendenhall went on to apply
his insight to a dizzying series of calculations. Using data from Tokyo seis-
mographs on the Japanese earthquake of 1887, he determined—though
admittedly only to within an order of magnitude—the “mechanical value of
a cubic mile of earthquake.” This, he showed, was equivalent to the free fall
of “a cube of rock one thousand feet on each edge, the mass of which would
be 75,000,000 tons, through a vertical distance of about 166 feet.” He even
took the time to translate that figure into horsepower.
88
As his delight in
these numbers makes clear, Mendenhall's definition of seismic intensity
as “the energy involved in an earthquake” was part of a nineteenth-century
obsession with measuring, harnessing, and exploiting natural sources of en-
ergy. But the significance of his calculations for the history of seismology is
more specific. What Mendenhall, Holden, Dutton, and their colleagues had
begun to do, in the guise of forging an objective language for seismology,
