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The Atomic Bombings of Hiroshima and Nagasaki by United States. Army. Corps of Engineers. Manhattan District
page 47 of 87 (54%)
blown out some time later (depending on their distance from X) by the
pressure blast which followed the flash radiation.



CALCULATIONS OF THE PEAK PRESSURE OF THE BLAST WAVE


Several ingenious methods were used by the various investigators to
determine, upon visiting the wrecked cities, what had actually been the
peak pressures exerted by the atomic blasts. These pressures were computed
for various distances from X, and curves were then plotted which were
checked against the theoretical predictions of what the pressures would be.
A further check was afforded from the readings obtained by the measuring
instruments which were dropped by parachute at each atomic attack. The
peak pressure figures gave a direct clue to the equivalent T.N.T. tonnage
of the atomic bombs, since the pressures developed by any given amount of
T.N.T. can be calculated easily.

One of the simplest methods of estimating the peak pressure is from
crushing of oil drums, gasoline cans, or any other empty thin metal vessel
with a small opening. The assumption made is that the blast wave pressure
comes on instantaneously, the resulting pressure on the can is more than
the case can withstand, and the walls collapse inward. The air inside is
compressed adiabatically to such a point that the pressure inside is less
by a certain amount than the pressure outside, this amount being the
pressure difference outside and in that the walls can stand in their
crumpled condition. The uncertainties involved are, first, that some air
rushes in through any opening that the can may have, and thus helps to
build up the pressure inside; and, second, that as the pressure outside
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