ther away from the
center of explosion. It seemed as though the great destruction caused
in the torpedo plant had weakened the blast a little, and the full
power was not restored for another 1,000 feet or more.

FLASH BURN

As already stated, a characteristic feature of the atomic bomb, which
is quite foreign to ordinary explosives, is that a very appreciable
fraction of the energy liberated goes into radiant heat and light. For
a sufficiently large explosion, the flash burn produced by this
radiated energy will become the dominant cause of damage, since the
area of burn damage will increase in proportion to the energy released,
whereas the area of blast damage increases only with the two-thirds
power of the energy. Although such a reversal of the mechanism of
damage was not achieved in the Hiroshima and Nagasaki bombs, the
effects of the flash were, however, very evident, and many casualties
resulted from flash burns. A discussion of the casualties caused by
flash burns will be given later; in this section will be described the
other flash effects which were observed in the two cities.

The duration of the heat radiation from the bomb is so short, just a
few thousandths of a second, that there is no time for the energy
falling on a surface to be dissipated by thermal defusion; the flash
burn is typically a surface effect. In other words the surface of
either a person or an object exposed to the flash is raised to a very
high temperature while immediately beneath the surface very little rise
in temperature occurs.

The flash burning of the surface of objects, particularly wooden
objects, occurred in Hiroshima up to a radius of 9,500 feet from X; at
Nagasaki burns were visible up to 11,000 feet from X. The charring and
blackening of all telephone poles, trees and wooden posts in the areas
not destroyed by the general fire occurred only on the side facing the
center of explosion and did not go around the corners of buildings or
hills. The exact position of the explosion was in fact accurately
determined by taking a number of sights from various objects which had
been flash burned on one side only.

To illustrate the effects of the flash burn, the following describes a
number of examples found by an observer moving northward from the
center of explosion in Nagasaki. First occurred a row of fence posts
at the north edge of the prison hill, at 0.3 miles from X. The top and
upper part of these posts were he