DATE: July 24, 1945
TO: Lt. Taylor
FROM: V. Weisskopf
SUBJECT: EYE WITNESS ACCOUNT
You have asked me to submit to you an eye witness account of the explosion. I was located at base camp and watched the phenomenon from a little ridge about 100 yds. east of the water tower. Groups of observers had arranged small wooden sticks at a distance of 10 yds. from our observation place in order to estimate the size of the explosion. They were arranged so that their distance corresponded to 1000 ft. at zero point. I looked at the explosion through the dark glass, but I have provided for an indirect view of the landscape in order to see the deflected light.
When the explosion went off, I was first dazzled by this indirect light which was much stronger than I anticipated, and I was not able to concentrate upon the view through the dark glass and missed, therefore, the first stages of the explosion. When I was able to look through the dark glass I saw flames and smoke of an estimated diameter of 1000 yds. which was slowly decreasing in brightness seemingly due to more smoke development. At the same time it rose slightly above the surface. After about three seconds its intensity was so low I could remove the dark glass and look at it directly. Then I saw a reddish glowing smoke ball rising with a thick stem of dark brown color. This smoke ball was surrounded by a blue glow which clearly indicated a strong radioactivity and was certainly due to the gamma rays emitted by the cloud into the surrounding air. At that moment the cloud had about 1000 billions of curies of radioactivity whose radiation must have produced the blue glow.
The first two or three seconds, I felt very strongly the heat radiation all over the exposed parts of my body. The part of my retina which was exposed to the indirect light from the surrounding mountains was completely blinded and I could feel traces of the after image 30 minutes after the shock.
The reddish cloud darkened after about 10 or 20 seconds and rose rather rapidly leaving behind a thick stem of dark brown smoke. After this, I remember having seen a white hemisphere rising above the clouds in continuation of the breakthrough of the explosion cloud through the ordinary cloud level.
The path of the shock wave through the clouds was plainly visible as an expanding circle all over the sky where it was covered by clouds.
After about 45 seconds the sound wave arrived and it struck me as being much weaker than anticipated.
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