on of an atomic bomb
and that of an ordinary T.N.T. bomb is of course in magnitude; as the
President announced after the Hiroshima attack, the explosive energy of
each of the atomic bombs was equivalent to about 20,000 tons of T.N.T.

But in addition to its vastly greater power, an atomic explosion has
several other very special characteristics. Ordinary explosion is a
chemical reaction in which energy is released by the rearrangement of
the atoms of the explosive material. In an atomic explosion the
identity of the atoms, not simply their arrangement, is changed. A
considerable fraction of the mass of the explosive charge, which may be
uranium 235 or plutonium, is transformed into energy. Einstein's
equation, E = mc^2, shows that matter that is transformed into energy
may yield a total energy equivalent to the mass multiplied by the
square of the velocity of light. The significance of the equation is
easily seen when one recalls that the velocity of light is 186,000
miles per second. The energy released when a pound of T.N.T. explodes
would, if converted entirely into heat, raise the temperature of 36
lbs. of water from freezing temperature (32 deg F) to boiling
temperature (212 deg F). The nuclear fission of a pound of uranium
would produce an equal temperature rise in over 200 million pounds of
water.

The explosive effect of an ordinary material such as T.N.T. is derived
from the rapid conversion of solid T.N.T. to gas, which occupies
initially the same volume as the solid; it exerts intense pressures on
the surrounding air and expands rapidly to a volume many times larger
than the initial volume. A wave of high pressure thus rapidly moves
outward from the center of the explosion and is the major cause of
damage from ordinary high explosives. An atomic bomb also generates a
wave of high pressure which is in fact of, much higher pressure than
that from ordinary explosions; and this wave is again the major cause
of damage to buildings and other structures. It differs from the
pressure wave of a block buster in the size of the area over which high
pressures are generated. It also differs in the duration of the
pressure pulse at any given point: the pressure from a blockbuster
lasts for a few milliseconds (a millisecond is one thousandth of a
second) only, that from the atomic bomb for nearly a second, and was
felt by observers both in Japan and in New Mexico as a very strong wind
going by.

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