SHIELDING, OR SCREENING FROM BLAST


In any explosion, a certain amount of protection from blast may be gained
by having any large and substantial object between the protected object and
the center of the explosion. This shielding effect was noticeable in the
atomic explosions, just as in ordinary cases, although the magnitude of the
explosions and the fact that they occurred at a considerable height in the
air caused marked differences from the shielding which would have
characterized ordinary bomb explosions.

The outstanding example of shielding was that afforded by the hills in the
city of Nagasaki; it was the shielding of these hills which resulted in the
smaller area of devastation in Nagasaki despite the fact that the bomb used
there was not less powerful. The hills gave effective shielding only at
such distances from the center of explosion that the blast pressure was
becoming critical - that is, was only barely sufficient to cause collapse -
for the structure. Houses built in ravines in Nagasaki pointing well away
from the center of the explosion survived without damage, but others at
similar distances in ravines pointing toward the center of explosion were
greatly damaged. In the north of Nagasaki there was a small hamlet about
8,000 feet from the center of explosion; one could see a distinctive
variation in the intensity of damage across the hamlet, corresponding with
the shadows thrown by a sharp hill.

The best example of shielding by a hill was southeast of the center of
explosion in Nagasaki. The damage at 8,000 feet from X consisted of light
plaster damage and destruction of about half the windows. These buildings
were of European type and were on the reverse side of a steep hill. At the