osion over Nagasaki affected an over-all area of
approximately 42.9 square miles of which about 8.5 square miles were
water and only about 9.8 square miles were built up, the remainder
being partially settled. Approximately 36% of the built up areas were
seriously damaged. The area most severely damaged had an average
radius of about 1 mile, and covered about 2.9 square miles of which 2.4
were built up.

In Nagasaki, buildings with structural steel frames, principally the
Mitsubishi Plant as far as 6,000 feet from X were severely damaged;
these buildings were typical of wartime mill construction in America
and Great Britain, except that some of the frames were somewhat less
substantial. The damage consisted of windows broken out (100%), steel
sashes ripped out or bent, corrugated metal or corrugated asbestos
roofs and sidings ripped off, roofs bent or destroyed, roof trusses
collapsed, columns bent and cracked and concrete foundations for
columns rotated. Damage to buildings with structural steel frames was
more severe where the buildings received the effect of the blast on
their sides than where the blast hit the ends of buildings, because the
buildings had more stiffness (resistance to negative moment at the top
of columns) in a longitudinal direction. Many of the lightly
constructed steel frame buildings collapsed completely while some of
the heavily constructed (to carry the weight of heavy cranes and loads)
were stripped of roof and siding, but the frames were only partially
injured.

The next most seriously damaged area in Nagasaki lies outside the 2.9
square miles just described, and embraces approximately 4.2 square
miles of which 29% was built up. The damage from blast and fire was
moderate here, but in some sections (portions of main business
districts) many secondary fires started and spread rapidly, resulting
in about as much over-all destruction as in areas much closer to X.

An area of partial damage by blast and fire lies just outside the one
just described and comprises approximately 35.8 square miles. Of this
area, roughly 1/6th was built up and 1/4th was water. The extent of
damage varied from serious (severe damage to roofs and windows in the
main business section of Nagasaki, 2.5 miles from X), to minor (broken
or occasionally broken windows at a distance of 7 miles southeast of X).

As intended, the bomb was exploded at an almost ideal location over
Nagasaki to do the maximum damage to i