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 falls, the air inside cannot escape
sufficiently fast to avoid the walls of the can being blown out again
to some extent. These uncertainties are such that estimates of
pressure based on this method are on the low side, i.e., they are
underestimated.
Another method of calculating the peak-pressure is through the bending
of steel flagpoles, or lightning conductors, away from the explosion.
It is possible to calculate the drag on a pole or rod in an airstream
of a certain density and velocity; by connecting this drag with the
strength of the pole in question, a determination of the pressure wave
may be obtained.
Still another method of estimating the peak pressure is through the
overturning of memorial stones, of which there are a great quantity in
Japan. The dimensions of the stones can be used along with known data
on the pressure exerted by wind against flat surfaces, to calculate the
desired figure.