- ^ - 615 



A=°iNOIX 



.WTES Olf PRACTICAL DETAILS OF METHOD 



1, A stanaara 35 m.";, Sinclair or Vinton camera nas been used in the tests at an average 

 film sceea of 25 f ramcs/secono. As largt a oome as oossiDle is required for satisfactory 

 analysis sa a orocor choice of lens jno flistance is imoortant. with a 5 inch lens, the camen 

 should oe about 700 to 1,000 yr-.ras from the imcact, for a dome diameter of 200 to 300 feet. 



2, The scale of the oomc h.is to De obfiined from the scnlf; of the eeroolane. Corrections 

 have to be acolisd for the aifferent ocsitlons of the aeroplane and the imoact, and for high 

 orccs, wnsn the aercolane is out :f tno oicture some time bef.re imcact occurs, these corrections 

 are imcortint. T; keeo the crrrections small, it is advisable to use ss l:ng a f;cal length 

 lens as Dossible, as this will enable the ground distance to be as large as oossible ay 

 comcarison with the height and forwara travel during the fall of the bomb. 



X D 



3, The measured dore heights arc corrected for obliquity. This correction is: -js- 



at the centre of the done, wh;rp D is the dome diameter, x is the height of the camera above 

 water level, and y is the distance between camera and dome. The correction is orooort ionately 

 less at raaii away from tht centr;. 



4, Both side? of the dome are measured and the mean is t^ken in the depth calculations. 

 From Figure 2 it will be seen that the variations are at times considerable. 



5, The table of reduction used to obt-,in the depth from the measurements is shown in Figure 5. 

 The exarcle worked out in the table is the same as thyt shown in Figures 1 and 2. In practice 

 instead of taking the ratios of the initial velocities at the various radii it has been found 

 more convenient to take the ratios of the dome heights corrected for the gravity fall. 



Equating these height ana velocity r^.tioa is only strictly justified when the time intervals are 

 small ana the 3eceler?.ti on ■^t the spray is orcDurt i ;nal to its velocity. The apprcximat ion 

 amounts to neglecting terms involving t^ ana higher powers, where t is the time interval from 

 detonation. It is realisea that the deceleration of the seray is more nearly Drooortional to 

 the square of its velocity, but the effect of botn approximations has been found to be negligible 

 In practice. The analysis of tne two drocs which gave the shallowest detonation has been 

 repeated using velocity ratios in place of the corrected height ratios; agreement was obtalnao 

 within less tnan 0.5 feet. 



