October 14, 1922] 



NA TURE 



5i5 



The photographing-spark-switch and battery-switch 

 inside the apparatus house are set, the lights turned out, 

 and the slide of the plateholder drawn. The operator 

 then leaves the apparatus house by means of a light- 

 tight passage and starts the static-machine motor by 

 closing the table switch. He then makes any necessary 



correction to the aiming and fires when the signal light 

 appears. The bullet leaves the rifle and on its way to 

 the apparatus house passes the trigger upon the dia- 

 phragm of which the sound wave impinges. This 

 immediately trips the photographing-spark-switch and 

 it starts to close the trigger gap in the spark circuit. 

 The bullet continues on past the trigger, entering the 

 apparatus house through a sheet of thin paper, used 

 to shut out the light, and arrives in front of the photo- 

 graphic plate, upon which it is then projected by the 

 photographing spark. The motor switch is then 

 opened and the slide replaced in the 

 plateholder, which may then be taken 

 to the dark room and developed. 



In case the speed of the bullets to 

 be photographed is not known, a 

 piece of paper or wire screen is placed 

 in the path of the bullet in front of 

 the photographic plate, and if a 

 puncture in the screen is shown when 

 the plate is developed, evidently the 

 bullet had gone past the plate before 

 the spark occurred. The trigger 

 must then be moved back from the 

 plate and the process repeated. Con- 

 tinuing in this manner the position 

 of the bullet when the spark occurs 

 will soon be bracketed within limits 

 sufficiently small, so that an observer 

 inside the apparatus house mav see 

 the bullet as the spark illuminates it. 

 Visual adjustment only is then used 

 until most of the bullets are seen in 

 the desired locality when the spark Fig. *.- 



occurs. 



The apparatus is provided with two light gaps, one 

 horizontal and the other vertical. This arrangement 

 facilitates the taking of two photographs of the same 

 bullet, a plan and elevation view. This is particularly 

 useful in investigating a projectile of an unstable char- 

 acter having a tendency to tumble, since from the two 

 \ iews its actual position in space may be constructed. 



The two coaxial waves which the bullet propagates 



NO. 2763, VOL. I 10] 



from its nose and base appear on the photograph 

 (Fig. 4) to have different slopes. This arises from the 

 projection, for while the axis of the wave is parallel to the 

 photographic plate, and therefore projected in propor- 

 tion to its length, the radius of the projected wave is 

 somewhat inclined to the plate and causes the distortion. 

 The true angle of the conical sound wave in air may, 

 however, be readily computed, from which the speed of 

 the bullet producing the wave may be determined. 1 



In obtaining the photographs of the Colt automatic 

 25-calibre pistol, reproduced in Figs, i, 2, and 3. the 

 interrupter trigger was removed from the circuit and 

 a timing device substituted, which consists of two 

 electromagnets connected in series and adjustable in 

 height, their function being to drop two steel balls at 

 the same time when the key opening their circuit was 

 depressed. One of these balls fell on a lever which 

 lulled the trigger T of the automatic pistol by exerting 

 tension on the arm A (see Fig. 1). The other ball, 

 released from a greater height at the same time as the 

 first, impinged on a device setting off the photographing 

 spark. By this means any reasonable lag or lead in 

 the firing of the pistol with respect to the occurrence of 

 the photographing spark could be obtained. The firing 

 apparatus is obviously not a precision instrument and 

 could, of course, be replaced by an accurate timing 

 device should the investigator require information of 

 such a character. 



The turbulent gases of the propelling charge are 

 clearly shown in Figs. 1, 2, and 3. All these photo- 

 graphs of the automatic pistol represent a stage in the 

 recoil relatively long after the ejection of the bullet 

 from the muzzle, since it will be seen that the receiver 

 R has reached the position of extreme recoil and the 

 empty cartridge case is being ejected in Figs. 2 and 3. 



If the height of the ball which triggers the spark is 

 changed progressively by some small known amount, a 

 series of photographs of such an automatic pistol al 

 slightly different calculable time intervals could be 

 secured. From the data obtainable from such photo- 

 graphs a pressure time curve could be computed. 



1 Journal of the Franklin Institute, May 1922. 



