In all six shots were fired, five of which were for the purpose of winning coal, while the sixth was expressly arranged as a "blowout shot." The roburite—which resembles nothing so much as a common yellow sugar—is packed in cartridges of about 4½ in. in length and 1½ in. in diameter, each containing about 65 grammes (one-seventh of a pound) inclosed in a waterproof envelope. By dividing a cartridge, any desired strength of charge can be obtained. The first shot had a charge of 90 grammes (one-fifth of a pound) placed in a hole drilled to a depth of about 4 ft. 6 in., and 1¾ in. in diameter. All the safety lamps were carefully covered, so that complete darkness was produced, but there was no visible sign of an explosion in the shape of flame—not even a spark—only the dull, heavy report and the noise made by the displaced coal. A large quantity of coal was brought down, but it was considered by most of the practical men present to be rather too much broken. The second shot was fired with a single cartridge of 65 grammes, and this gave the same remarkable results as regards absence of flame, and, in each case, there were no noxious fumes perceivable, even the moment after the shot was fired. This reduced charge gave excellent results as regards coal winning, and one of the subsequent shots, with the same weight of roburite, produced from 10 to 11 tons of coal in almost a solid mass.
It has been found that a fertile cause of accidents in coal mines is insufficient tamping, or "stemming," as it is called in Yorkshire. Therefore a hole was bored into a strong wall of coal, and a charge of 45 grammes inserted, and very slightly tamped, with the view of producing a flame if such were possible. This "blowout" shot is so termed from the fact of its being easier for the explosion to blow out the tamping, like the shot from a gun, than to split or displace the coal. The result was most successful, as there was no flash to relieve the utter darkness.
The second set of experiments took place on October 24 last, in the Monk Bretton colliery, near Barnsley, of which Mr. W. Pepper, of Leeds, is owner. This gentleman determined to give the new explosive a fair and exhaustive trial, and the following programme was carried out in the presence of a very large gathering of gentlemen interested in coal mining. The chief inspector of mines for Yorkshire and Lincolnshire, Mr. F.N. Wardell, was also present, and the Roburite Explosives Company was represented by Lieut.-General Sir John Stokes, K.C.B., R.E., chairman, and several of the directors.
1. Surface Experiments.—A shot fired on the ground, exposed. This gave no perceptible flame (70 grammes of roburite was the charge in these experiments).
2. A shot fired on the ground, bedded in fine coal dust. No flame nor ignition of the coal dust was perceptible.
3. A shot fired suspended in a case into which gas was conducted, and the atmospheric air allowed to enter so as to form an explosive mixture. The gas was not fired.
4. A shot fired in a boiler flue 16 ft. by 2 ft. 8 in., placed horizontally, in which was a quantity of fine coal dust kept suspended in the air by the action of a fan. No flame nor ignition of the coal dust took place.
5. A shot fired as above, except that an explosive mixture of gas and air was flowing into the boiler tube in addition to the coal dust. That this mixture was firedamp was proved by the introduction of a safety lamp, the flame of which was elongated, showing what miners call the "blue cap." There was no explosion of the gas or sign of flames.
6. A shot of roburite fired in the boiler tube without any gas or suspended coal dust. The report was quite as loud as in the preceding case; indeed, to several present it seemed more distinct.
7. A shot of ½ lb. gunpowder was fired under the same condition as No. 5, i.e., in an explosive mixture of gas and air with coal dust. The result was most striking, and appeared to carry conviction of the great comparative safety of roburite to all present. Not only was there an unmistakable explosion of the firedamp, with very loud report, and a vivid sheet of flame, but the gas flowing into the far end of the boiler tube was ignited and remained burning until turned off.