EMENDATIONS

 

 

 

In the year that "Hear No Evil" has been on the web, numerous individuals have been kind enough to offer suggestions to improve the accuracy of the analysis. Two points require significant emendation as discussed below.

 

Shock Wave Arrival Time. -

 

The first correction concerns the theory by Luis Alvarez that the blur in the Zapruder film at frame 313 could have been caused by the shock wave of a gunshot impinging on the camera body. The blur analysis of the film was based on the premise that Zapruder would have been startled by the sound of a gunshot, his reaction causing a sudden angular movement of the camera, which would then be detectable as blur. The problem is that the strongest angular motion detectable in the Zapruder film is at frame 313, coincident with the impact of the fatal bullet. The sound of a gunshot fired from the sniper's nest in the Book Depository and striking JFK at frames 312-313 will not arrive at Zapruder's position until about frame 315. Likewise, the shockwave from a bullet on a path from the book depository to JFK will not arrive at Zapruder's position until after frame 313 is exposed. However, the calculations provided in HNE in support of this assertion were incorrect.

 

The shock wave arrives at Zapruder's position ahead of the sound of the muzzle blast because the sound wave must travel a distance of 270 ft on a direct path to reach Zapruder, and at the speed of sound (1123 fps at 65F) this time is 240 msec. The shock wave emanates from the nose of the bullet and although the shockwave must travel a longer path to reach Zapruder, it travels at twice the speed of sound during the longer leg of its approach. As the shock wave is shed from the bullet it quickly returns to its natural speed in air, which is the speed of sound, for its last leg towards Zapruder. I erroneously assumed that the front of the shock wave reaching Zapruder would emanate from the point on the bullet's path closest to Zapruder, which would have been just before striking JFK. However, Physicist Paul Hoch has pointed out that because the angle of approach towards Zapruder is increasing, the speed of approach towards Zapruder is decreasing, and becomes less than the speed of sound at some point upstream from the target. It is from there that the shock wave front will emanate towards Zapruder. By calculating the arrival of the shock wave from a series of points along the bullets path at 10 ft intervals, I find that Hoch is correct. The shock wave, which reaches Zapruder, emanates from a point approximately 210 ft downstream from the muzzle.

 

In redoing these calculations a second error was discovered. A witness for the HSCA estimated that the impact velocity of the fatal bullet would have been around 1600 fps, and I employed that value in my earlier calculations. However, Alfred Olivier provided more precise values of the bullet’s velocity in his testimony to the Warren Commission. The muzzle velocity of Oswald's rifle was 2160 fps. The bullet's velocity at 60 yds (180 ft) was 1904 fps. The bullet slows down due to air resistance; however, at the rate of slowing measured by Olivier the bullet's velocity at 265 ft would have been significantly greater than the HSCA's estimate of 1600fps. This was confirmed by checking ballistic tables for similar bullets (the 6.5 mm Carcano bullet weighed 160 grains). For example, a Winchester .30 caliber 170-grain bullet with an MV of 2200 fps has a velocity of 1900 fps at 100 yds (300 ft). A Remington .35 caliber, 200-grain bullet at MV 2080 fps, has a velocity at 300 ft of 1800 fps. A Remington .27 cal 120-grain bullet with the same MV of 2080 fps also has the same velocity at 300 ft of 1800 fps. Subsequently I found the value of 1800 fps adopted by Luis Alvarez in his studies of the ballistics in this case (Alvarez 1976, Am. J. Phys. p. 821) and this value, if not exact, is at least coherent with the measurements of Olivier.

 

The table below shows the shock wave arrival times from different points in the trajectory and the values used to obtain the times. D1 is the distance from the muzzle in feet; D2 is the distance from the end point in D1 to Zapruder. V1 is the muzzle velocity (in fps); V2 is the velocity at the end point of D1. Vm is the mean velocity for D1, which is V1+V2/2. T1 is the time for the bullet to travel D1, which is D1/Vm. T2 is the time for the shock wave to travel from the end point of D1 to Zapruder. T1+T2 is the time at which the shock wave arrives at Zapruder after trigger time (all times in milliseconds).

 

D1     D2      V1      V2     Vm      T1     T2       T1+T2

 

180    109    2160   1904   2032     89     97      186 msec

190    102    2160   1889   2025     94     91      185 msec

200      93    2160   1875   2018     99     85      184 msec

210      88    2160   1862   2011   104     78      182 msec

220      83    2160   1852   2006   110     74      184 msec

 

 

From this table it can be seen that the shock wave emanates from a point approx. 210 ft downstream from the muzzle and reaches Zapruder 182 msec after trigger time.

 

The mean velocity of the bullet between the sixth floor and JFK would have been (2160+1800)/2 = 1980 fps. At this speed the impact time of the bullet would be at 265/1980 = 134 msec after trigger time. Therefore, the shock wave arrives at Zapruder 48 msec after impact time. For the reasons explained in HNE, impact time was approx. 15 msec before the exposure of Z-313 begins. Because the exposure time is approx. 27 msec, Z-313 exposure ends at 42 msec after impact and 6 msec before the shock wave arrives. Thus, even if the shock wave were supposed to be strong enough to cause a jerky motion in Zapruder's camera body, it does not arrive in time to cause the angular motion evident at Z-313.

 

 

 

 

Position of Car-8. -

 

In the 20th frame of the third sequence of Robert Hughes' film of the President's motorcade, H.B. McLain's motorcycle is seen at the intersection of Houston and Main Streets. McLain is the only motorcycle patrolman that could have been at the acoustically predicted locations to capture the sounds of the assassination gunfire. In HNE it was estimated that H20 correlated to a time approx. 6-7 sec before the first shot. This estimate relied on two values; one was an estimate of the average speed of the eighth car in the motorcade of approx. 7 mph. The other was the distance of Car-8 between where it is seen in H20 and where it is seen in Zapruder frame 220. In HNE I estimated this distance to be 88 ft.

 

The position of Car-8 in the Zapruder film can be determined by a line of sight from Zapruder's position through a small oak tree in Dealey Plaza, both of which can be fixed accurately, and which line intercepts the front bumper of Car-8 at frame 220. Michael Russ has suggested that a similar line of sight could be applied to the Hughes film to fix the earlier position of Car-8. Such an analysis is problematic because, unlike the Zapruder case, Hughes' position is not known precisely. In a photograph taken by Charles Bronson just as the President's motorcade arrived at the intersection one can see the crowd of people where Hughes must be standing. Hughes has to be standing among this crowd because were he standing further back he would not have had an unobstructed view of the motorcade as seen in the film. As the motorcade approached, Hughes' aimed his camera east down Main Street. His line of sight indicates that he was standing almost exactly in line with the road stripe separating the eastbound lanes of traffic, or slightly to the north of this line. As he panned to his left following the motorcade he eventually pointed his camera north on Houston Street. From this line of sight it can be seen that he was standing almost exactly in the middle of the western most lane of traffic, or judging by the line of grease spots, slightly to the west. The intersection of these two lines gives us a fair approximation of Hughes' position. Assuming the traffic lanes were 9 ft wide, this position can be defined as 8 ft south of the center line of Main Street and 14 ft west of the center line of Houston Street. We have to assume this width because the lines have been repainted since 1963. In 1963 Houston was a two way street with four lanes of traffic. Today it is a one-way street with three lanes of traffic. By north and south I am referring to the general orientation, as the streets of Dallas are not laid out in the cardinal compass directions. I have plotted Hughes' position on a topographic map of Dealey Plaza. I have also plotted a line of sight from Hughes to the corner of the Records Building because Car-8 lies on this line of sight. Also, the left front corner of Car-9 just intercepts this line. From Hughes' viewpoint the view of Car-8 was partially blocked by Car-9 as shown in the juxtaposition on the map. The Hughes photo shows an offset in that the left side of Car-8 aligned with the midstripe on Houston Street, whereas Car-9 was about 2-3 ft to the east of the line. As a further corroboration for the plotted location I have drawn in the crosswalk stripe, which can be seen about half a car-length behind Car-9 in H20. My error in the corresponding map in HNE was to misplace this stripe too far south (there were actually 3 stripes). The correct position of the stripe is in line with the first window of the Court House. With this corrected position for Car-8 the distance from its position in Z-220 (front bumper to front bumper) is 73 ft, about a car-length further north than its position plotted earlier (the plot provided is diagrammatic, the actual measurement was made on the published HSCA survey map with proportional cut-outs representing the cars which were 15x7 ft in dimension).

 

In order to correlate H20 to the Zapruder film we need an estimate of the speed of Car-8. In the Zapruder film Car-8 has a velocity of 8.5 mph. However, one cannot apply this rate of speed to the entire interval between H20 and Z-220 because the speed of the motorcade was known to vary. For example, Car-5 can be seen rounding the corner in the Zapruder film at about this same time and it is traveling at only 5.5 mph. This indicates that the cars would slow down for the turns and then speed up in the straight-aways. This would cause an accordion effect in the motorcade with the cars always speeding up or slowing down as the drivers reacted to the cars ahead. Because Car-8 seen in the Zapruder film is in the middle of the straightaway it was most likely at or close to maximum speed. Therefore the most likely average speed for Car-8 between Z-220 and H-20 would have been around 6-8 mph. This corresponds to a speed of 8.8 to 11.7 fps. Therefore, the time lapse to cover the distance of 73 ft between Z-220 and H20 would be in a range from 6.2 to 8.3 sec. The first shot correlated to Zapruder frame 175, which is 2.4 sec prior to Z-220 (at 18.3 frames/sec). Therefore, from his position at the intersection of Houston and Main, McLain had approx. 3.8 to 5.9 sec to reach the first acoustically identified position 180 ft away. The 5.9 sec estimate requires McLain to travel at an average speed of 20.8 mph. The 3.8 sec timeframe would require a speed of 32 mph. A middle value of 5 sec requires a speed of 25 mph.

 

Another film gives support to the inferred speeds. In H20, McLain's motorcycle is close to Car-10 at the intersection of Houston and Main. When Car-10 rounded the intersection at Elm and Houston, Malcolm Couch, a newsreel photographer in Car-10, aimed his camera down Elm Street and filmed McLain approx. 200 ft away to the west. This means that in the time that it took Car-10 to travel 200 ft from Main to Elm, McLain had traveled 400 ft. Therefore his average speed during the interval has to have been twice whatever Car-10 was doing. In this newsreel Cars-8, 9 and 10 are only about a car length apart, just as they were in the Hughes film. If the cars averaged 7 mph on Houston Street, then McLain had to average 14 mph during the same interval. However, this speed has to be apportioned in accordance with the acoustical data which indicates a faster speed prior to the shots and then an idle speed after the shots. Seconds before the first sound acoustically identified as a gunshot occurs, the motorcycle motor noise diminished by 75%, indicating that the driver let off the accelerator, presumably in anticipation of making the turn around Elm and Houston. For the next 30 sec the motor continued at idle. The acoustical data require a speed of 11-12 mph during the shooting when the motorcycle would have to be rounding the corner. Thus, a required speed of 20-25 mph for McLain on Houston Street is in accord with both the acoustical and the filmed evidence. It should be understood that these values are soft values, which is to say, they are inferences from observed data, not actual measurements, and therefore do not prove that McLain was in the right position to record the shots, only that he reasonably could have been.

 

 

Donald Thomas

January 2003

 

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