294 Al!^KrUAL. REPORT SMITHSONIAN INSTITUTION, 1910. 



concrete barricades are erected on each side of the mouth of the 

 gallery, and a thick iron plate is so suspended on a frame across, but 

 at a distance of 50 feet from, the mouth of the gallery, that it may so 

 swing as to deflect the blast and arrest any flying stemming or other 

 material which may be blown out of the gallery. This arrangement 

 is shown in plate 2, which also shows the reenforced-concrete founda- 

 tion in which the gallery is set and specimens of the " cannon " used. 

 The violence of the gallery explosions initiated by the charges of 

 explosive used may be judged from plate 3, which is a photograph of 

 an explosion of a coal-dust-air mixture. 



The observation room from which the charge is fired and the 

 visible phenomena occurring in the gallery observed is shown in 

 plate 4. This room is 40 feet long by 9 feet 5 inches wide, built with 

 brick walls 18 inches thick, and provided with a heavy plate-glass 

 window, 37 feet long by 6 inches wide, which is protected by two pro- 

 jecting wooden guards. This room with its window is made so large 

 in size that it may not only provide a large field of sight, but that 

 it may accommodate a considerable number of persons, for the station 

 is designed to be educational as w^ell as experimental, and coal miners 

 are brought there in large numbers to be convinced by experimental 

 demonstrations of the accidents that may arise unless they use the 

 explosives recommended by the station, and use them in the pre- 

 scribed manner and amount. 



Prior to testing the explosives in the gallery, which is both costly 

 and time consuming, they are subjected to other tests which may 

 show them to possess such characteristics as to render them unfit for 

 use and render the gallery test unnecessary. Thus they are inspected 

 physically and analyzed chemically, and an admirably equipped and 

 well-manned laboratory is provided for this purpose. Among other 

 tests, the gravimetric density of the material, in the original package 

 in which it would be used in the mine, is determined by the aid of 

 dry sand as the mobile medium, and this density is carefully pre- 

 served in those tests in which density is a factor. 



One of the first tests to be made is the determination of what is 

 styled the " unit-disruptive charge," which is ascertained by the aid 

 of the ballistic pendulum shown in plate 5. This apparatus consists 

 essentially of two parts — the " cannon," in which the charge is fired, 

 and the pendulum, which receives the impact of the products of the 

 explosion, and that of the stemming when the latter is used. The 

 " cannon " is identical in form, construction, and proportions with 

 those used in the gallery. It is mounted on a four-wheeled truck, 

 to which it is made fast by straps and rods. The truck runs on a 

 30-inch track which is provided with a recoil bumper placed 9 feet 

 from the face of the pendulum mortar. The " cannon " is carefully 

 placed axially in line with the pendulum mortar. 



