The coping must be broken. Pencroff, directed by the engineer, attacked its outer facing vigorously. The hole which he made with his pick began under a horizontal edge of the bank, and penetrated obliquely so as to reach a level lower than the lake’s surface. Thus the blowing up of the rocks would permit the water to escape freely and consequently lower the lake sufficiently. The work was tedious, as the engineer, wishing to produce a violent shock, had determined to use not less than two gallons of nitro-glycerine in the operation. But Pencroff and Neb, taking turns at the work, did so well, that by 4 o’clock in the afternoon it was achieved. Now came the question of igniting the explosive. Ordinarily, nitro-glycerine is ignited by the explosion of fulminated caps, as, if lighted without percussion, this substance burns and does not explode. Smith could doubtless make a cap. Lacking fulminate, he could easily obtain a substance analogous to gun-cotton, since he had nitric acid at hand. This substance pressed in a cartridge, and introduced into the nitro-glycerine, could be lighted with a slow match, and produce the explosion. But Smith knew that their liquid had the property of exploding under a blow. He determined, therefore, to make use of this property, reserving the other means in case this experiment failed. The blow of a hammer upon some drops of the substance spread on a hard stone, suffices to provoke an explosion. But no one could give those blows without being a victim to the operation. Smith’s idea was to suspend a heavy mass of iron by means of a vegetable fibre to an upright post, so as to have the iron hang directly over the hole. Another long fibre, previously soaked in sulphur, was to be fastened to the middle of the first and laid along the ground many feet from this excavation. The fire was to be applied to this second fibre, it would burn till it reached the first and set it on fire, then the latter would break and the iron be precipitated upon the nitro-glycerine. The apparatus was fixed in place; then the engineer, after having made his companions go away, filled the hole so that the fluid overflowed the opening, and spread some drops underneath the mass of suspended iron. This done, Smith lit the end of the sulfured fibre, and, leaving the place, returned with his companions to the Chimneys. Twenty-five minutes after a tremendous explosion was heard. It seemed as if the whole island trembled to its base. A volley of stones rose into the air as if they had been vomited from a volcano. The concussion was such that it shook the Chimneys. The colonists, though two miles away, were thrown to the ground. Rising again, they clambered up to the plateau and hurried towards the place. A large opening had been torn in the granite coping. A rapid stream of water escaped through it, leaping and foaming across the plateau, and, reaching the brink, fell a distance of 300 feet to the shore below. CHAPTER XVIII.