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.