306 ANNUAL REPORT SMITHSONIAN INSTITUTION 1910. 



stack, is provided with a door in the side, through which the " can- 

 non" can be loaded, and with a vertical slit 8 feet in length b V 2 

 inches in width, which is so placed that its vertical center Coincides 

 with that of the conduit and also with that of the lens by which it is 

 viewed. At the time of firing the top of the stack is covered with 

 black paper. The conduit is closed at the point where it ends in the 

 wall of the dark room by a shutter. 



The camera consists of a drum on which the sensitized film is 

 mounted, an electric motor by which the drum is revolved at a known 

 rate, a quartz lens, a semicircular shield in which a stenopaic slit has 

 been cut, and a shutter by which to control the slit in the shield All 

 of these except the motor are inclosed in a light-tight box The 

 semicircular shield is placed close to and concentric with the drum 

 to prevent any light reaching the film except that passing through 

 the stenopaic slit. A lens of quartz is used because it focuses not 

 only the visible light rays, but also those invisible violet rays which 

 occur to a large extent in the flames from explosives. 



By means of a tachometer both the number of revolutions per 

 minute of the motor and the peripheral speed of the drum are 

 directly read off. The maximum peripheral speed of the drum is 

 20 meters per second, and this rate is emploved when detonating ex- 

 plosives are tested, but with slow-burning explosives the drum is 

 run at a. slower rate. At the 20-meter rate 1 millimeter width of 

 flame equals 0.05 millisecond of time, and as the measurements of 

 the flame photographs are read to the nearest quarter of a millimeter 

 the smallest time interval measured is the 0.0125 millisecond The 

 charge of explosive used in the test is 100 grams, and these charges 

 are fired both with and without stemming. 



The result of this test on black blasting powder and on a permis- 

 sible explosive are shown in plate 12, figure 1. By the term " per- 

 missible explosive » is meant an explosive which has satisfactorily 

 passed all the prescribed tests at the Pittsburg testing station and 

 is regarded as suitable for use in coal mines. 



One of the most novel of modem tests is that devised by J 

 Thomas, 1 who has employed the X rays for ascertaining the condi- 

 tion of the powder core in Bickford or running fuse. The cause of 

 misfires and delayed ignitions has been the subject of much specula- 

 tion, and among other theories proposed was that of a break in the 

 continuity of the powder cores. In plate 12, figure 2, which is a copv 

 of Thomas s X-ray picture, the interruption of continuity in two 

 pieces of the fuse shown is very apparent, 



1 J. Chem. Met. Soc. S. Africa 9, 183 ; 1008. 



