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This line rose like the top a curtain until it dis

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Thisline rose like the top ()! a curtainuntil it dis-appearedat the top of the ball at about11 ms. Shortlyafter the spikes struck the ground (alx)ut 2ms) there appearedon the groundaheadof (he shockwavea wide skirt of l~lnlpy matterandwit hin and above the skirt a smoothbelt (interpretedas the Mach wa~’e). (~riginally brighter thanthe main wave butrapidlygrowingdimmer.Twosuccessivevisiblefronts clrop})ed behindthewell-definedshock wave. The brighterbut less sharplylimitedball of fire fell l)ehind it at alxmt16 ms ( 105 m). At about 32 ms (144 m) there appearedimrnediat ely behindthe shock wave a darkfr(mt of absorbingmatter,whichtraveledslowlyout until it becameinvisibleat 0.85 s (375 m).The shock wave itself becameinvisibleat about O.10s (2.4 x 102 m) but was followedthereafter t{)0.39 s (460 m), firstbVits light-refractingpropertyand later by the momentumit impartedto aball(xmcable.Thehall of fire grew evenmore slowlyto a radiusof about3 x 102 m, until the ciust chmclgrowing out of the skirt almost envelopedit. The top of the ball started to rise again at 2s. At :1.5sa minimumh~wizontal diameter,or neck, appearedone-thirdoft he way up the skirt. and the por-tion of the skirt abovethe neck formeda vortexring. The neck narrowed,and the ring and fast-growingpile of matter aboveit rose as a new cloudof smoke,carryinga convectionstem of dustbehindit. A boundarywithin the cloud.betweenthe ring and the upper part. persisted for at least22 s. The stem appearedtwistedlike a left-handedscrew. Thecloud of smoke,surroundedI)y afaint purple haze, rose wit h its top travelingat 57 m/s, at least until the top reached1.5 km. Thelater historyof the cloudwas not quantitativelyrecorded.Data not shown in Fig. 9 includequantitativemeasurementson the refractionof light and thematerial \’elocitv behind the shock front, in certainintervals;the former can be made to yield themat erial density as a functionof radius behindthe shock front. 169.3Analysisof the EmittedLightFor the analysisof the emittedlight,we have densityreadingson motion-picturenegatives,quartz-prismspectrogramsfor the first few millisecondswit h time resolut ion oft he order of 10‘5s60
and for the first 1/5 s with lower resolution,photocellrecords (partlyusable)of the light intensityfor the first second,and thermopilerecords showing that the total radiant energy density receivedat 104 yd was 1.2 x 107ergs cm–2+ -15’ZO.Thefollowingobservations,amongothers,seemto deservespecialnotice.During the earliest stages observedby us (radius a 10 to 100 m) the shock wave radius followedTaylor’stwo-fifthspower law: radiustimes2/5.The shock wave was markedlydeformedby the platform;moreover,the radius in other direc-tions was influencedby the presence of the platform. 5A skirt of hot, lumpymatter,thus far unexplained,rose from the groundahead of the Machwave.

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