This line rose like the top a curtain until it dis

This line rose like the top ()! a curtain until it dis-
appeared at the top of the ball at about 11 ms. Shortly after the spikes struck the ground (alx)ut 2
ms) there appeared on the ground ahead of (he shock wave a wide skirt of l~lnlpy matter and
wit hin and above the skirt a smooth belt (interpreted as the Mach wa~’e). (~riginally brighter than
the main wave but rapidly growing dimmer. Two successive visible fronts clrop})ed behind the
well-defined shock wave. The brighter but less sharply limited ball of fire fell l)ehind it at alxmt
16 ms ( 105 m). At about 32 ms (144 m) there appeared imrnediat ely behind the shock wave a dark
fr(mt of absorbing matter, which traveled slowly out until it became invisible at 0.85 s (375 m).
The shock wave itself became invisible at about O.10s (2.4 x 102 m) but was followed thereafter t{)
0.39 s (460 m), first
bV
its light-refracting property and later by the momentum it imparted to a
ball(xm cable. “
The hall of fire grew even more slowly to a radius of about 3 x 102 m, until the ciust chmcl
growing out of the skirt almost enveloped it. The top of the ball started to rise again at 2s. At :1.5s
a minimum h~wizontal diameter, or neck, appeared one-third oft he way up the skirt. and the por-
tion of the skirt above the neck formed a vortex ring. The neck narrowed, and the ring and fast-
growing pile of matter above it rose as a new cloud of smoke, carrying a convection stem of dust
behind it. A boundary within the cloud. between the ring and the upper part. persisted for at least
22 s. The stem appeared twisted like a left-handed screw. The cloud of smoke, surrounded I)y a
faint purple haze, rose wit h its top traveling at 57 m/s, at least until the top reached 1.5 km. The
later history of the cloud was not quantitatively recorded.
Data not shown in Fig. 9 include quantitative measurements on the refraction of light and the
material \’elocitv behind the shock front, in certain intervals; the former can be made to yield the
mat erial density as a function of radius behind the shock front. 16
9.3 Analysis of the Emitted Light
For the analysis of the emitted light, we have density readings on motion-picture negatives,
quartz-prism spectrograms for the first few milliseconds wit h time resolut ion oft he order of 10‘5s
60

and for the first 1/5 s with lower resolution, photocell records (partly usable) of the light intensity
for the first second, and thermopile records showing that the total radiant energy density received
at 104 yd was 1.2 x 107ergs cm–2 + - 15’ZO.
The following observations, among others, seem to deserve special notice.
●
During the earliest stages observed by us (radius a 10 to 100 m) the shock wave radius followed
Taylor’s two-fifths power law: radius times 2/5.
●
The shock wave was markedly deformed by the platform; moreover, the radius in other direc-
tions was influenced by the presence of the platform. 5
●
A skirt of hot, lumpy matter, thus far unexplained, rose from the ground ahead of the Mach
wave.