METHODS OF TESTING EXPLOSIVES MUNEOE. 293 



charged is measured by a meter which can be read to one-twentieth 

 of a cubic foot. The air and gas are mixed by means of circulating 

 systems exterior to the gallery, operated by monogram exhausters. 

 The circulating system for the first division is stationary and includes 

 steam heating coils by which to bring the mixture in the gallery to a: 

 constant temperature. The remaining divisions of the gallery are 

 served by a portable device. 



When coal dust is to be used it is spread on a series of shelves, 

 20 feet long by 4 inches in width, lining the gallery, there being four 

 of these on each side, and in addition upon a steel trestle, having a 

 surface 20 feet by 12 inches, which is placed for that purpose in the 

 first section of the gallery when coal dust is to be used. This dust is 

 ahvays freshly ground from lump coal to 100 mesh in fineness just 

 before using. 



In addition the gallery is provided with a humidifying apparatus 

 provided with a Koerting exhauster, having a capacity of 240,000 

 cubic feet of free air per hour, by which the effect of moisture in 

 preventing the propagation of explosions may be quantitatively 

 ascertained. 



The explosives to be tested may be suspended in the chamber and 

 fired in the prepared atmosphere, and this method has been pursued 

 at some stations, but the regular practice at the Pittsburg testing 

 station is to fire the charge in a special " cannon," as this more nearly 

 simulates the conditions in mining where the charge is fired in a 

 bore hole in coal or rock. These " cannon " are cylinders 24 inches 

 in diameter by 36 inches in length, with bore holes 2^ inches in 

 diameter by 21^ inches in depth. The simplest of them have been 

 made in one piece from a low-carbon steel or nickel-steel forging. 

 Others have been built up from centrally perforated jackets of cast 

 steel, vanadium steel or other iron alloy, and a liner of nickel steel 

 or other metals or alloys. In repairing, after erosion, the liner has 

 been formed by the thermite process. No definite conclusions have 

 yet been reached as to the relative merits of the different forms of 

 construction. These " cannon " are shown in the right foreground 

 of plates 2 and 4 and in the center of plate 6, figure 1. 



The " cannon " is embedded in the concrete head of the gallery and 

 is so laid that its axial line coincides with the axial line of the gallery. 

 The " cannon " is loaded from within the gallery, but the charges 

 are fired by electric detonators for high explosives and electric 

 igniters for explosives of the gunpowder class, the firing machine 

 being located in an observation room GO feet distant from the gallery. 

 The larger part of the explosives tested are detonated, and they are 

 fired both stemmed and unstemmed into the sensitive mixtures. 



As when the explosive mixtures in the gallery are fired the blast 

 from the mouth of the gallery is very destructive in its effects, two 



