The application of Electricity to the Blasting of Rocks.
Perhaps the greatest modern improvement that has been made in blasting rocks has been by the introduction of the galvanic battery. It is well known that by closing the circuit of a voltaic current by means of thin platinum wire, or by fine iron or steel wire, the platinum becomes red-hot, and the iron or steel becomes instantly fused. All, therefore, that is necessary is to connect the two terminal wires of a voltaic battery by means of a fine wire of platinum or iron, and to bury this in gunpowder contained in a tin canister, or a fuse connected with a deposit of gunpowder. This was the method adopted by Colonel Pasley in removing the Royal George, which lay sunk at the bottom of the water at Spithead. Canisters of gunpowder, sometimes to the extent of three thousand pounds’ weight, were employed, and securely deposited in the sunken vessel, by workmen who descended in the diving-bell; the terminal wires of the battery, connected as above stated, having been previously inserted in the canisters, and these wires being extended to a great distance, the explosion took place the instant they were connected with the voltaic battery. After the vessel was thus blown to pieces by repeated explosions, divers descended to clear away the wreck, and to attach guns, &c., to chains let down from a ship above, and which were then hauled up by means of a crane.
Mr. Morgan, in the American Journal of Science, describes a fuse or cartridge which he has used with success in connexion with the voltaic battery. This cartridge is prepared by joining two pieces of clean copper wire to the ends of a fine steel wire, about one quarter of an inch in length, by means of waxed silk; a thin piece of wood is then spliced to both copper wires, to protect the steel wire from accidents, and to enable the maker to introduce it easily into a quill or small paper tube, which is to form the cartridge. This tube is filled with fine gunpowder, and made air and water-tight. Another piece of wood is then attached to this arrangement, and one of the copper wires is bent over so as to form an angle with the straight wire.
When it is required to use this cartridge, the copper wires are rubbed with sand-paper, and twisted round the wires of the voltaic battery. The cartridge is then placed deep in the hole made to receive the gunpowder, and the charge is fired from any distance.
Mr. Morgan found this arrangement very useful in removing stumps of trees; but one of his applications of it was curious and novel: he exploded some powder in a pond at the depth of ten feet, with the battery at the distance of two hundred and ten feet; the explosion, which was instantaneous, had the effect of killing a large eel; and “I have no doubt,” says Mr. Morgan, “that wild-fowl will yet be killed by means of shells placed at low-water on the banks where they feed; and by means of long connecting wires, the shells can be made to explode simultaneously among the birds.”
But the grandest application of gunpowder and the voltaic battery to the blasting of rocks, was made in the month of January, 1843, at Dover. It was determined by these means to attempt the removal of an enormous mass of the cliff facing the sea, which formed an obstruction to the line of railroad. A portion of the cliff which was penetrated by the tunnel made through Shakspeare’s Cliff gave way, about two years previously. About fifty yards of the tunnel were carried away, and a clear space was thus formed for the line of railroad, with the exception of a projecting point, which, prior to the slip alluded to, was the extremity of the part of the cliff pierced by the tunnel, and to remove which was the object of the operation in question.
To clear away this mass by the tedious process of manual labour, would have cost above twelve thousand pounds; and this consideration, as well as the time that would have been lost, induced Mr. Cubitt, the engineer, to try the bold expedient of blowing it away with gunpowder. “It cannot be denied,” remarks Captain Stuart, whose account of this great engineering operation we follow, “that there was apparent danger in the undertaking, for the weight of the mass to be removed was estimated at two million tons, and the quantity of powder used was more than eight tons, or eighteen thousand pounds. The quantity used in blowing up the fortifications of Bhurtpore was twelve thousand pounds, and this is said to have been the greatest explosion that had ever previously taken place for any single specific object.”
The front of the projection was about one hundred yards wide; this front was pierced with a tunnel about six feet in height, and three in breadth; three shafts, equidistant from each other, and from the entrances to the tunnel, were sunk to the depth of seventeen feet, and galleries were run, one from each shaft, parallel with each other, and at right angles with the line of the tunnel. These galleries varied in length, the longest having been twenty-six feet, and the shortest twelve feet, and at their extremities chambers were excavated in a direction parallel with the tunnel. This description will be the better understood by reference to the following figure.
1. The tunnel. 2. The shafts. 3. The galleries. 4. The chambers.
In the chambers, the powder was deposited in three nearly equal quantities; it was done up in fifty-pound bags, and the proportion in each chamber was contained in a wooden case, nearly as large as the chamber itself. Ignition was communicated by means of a voltaic battery; the conducting wires, one thousand feet in length, were passed over the cliff, one to each chamber, and the electricity was communicated in a shed built for the purpose on the top of the cliff, about fifty yards from the edge. The explosion was conducted by Lieutenant Hutchinson, R.E., who was engaged with General Pasley in blowing up the wreck of the Royal George. The time appointed for the explosion to take place, was two o’clock P.M., 26th January, 1843, the tide being then at its lowest ebb. The arrangements, to preserve order and prevent danger, were good. A space was kept clear by a cordon of artillery, and the following programme was issued:—
“Signals, January 26, 1843.
“1st. Fifteen minutes before firing, all the signal flags will be hoisted.
“2nd. Five minutes before firing, one gun will be fired, and all the flags will be hauled down.
“3rd. One minute before firing, two guns will be fired, and all the flags (except that on the point which is to be blasted) will be hoisted up again.”
These signals were given exactly at the specified times, and when the expected moment arrived, a deep subterranean sound was heard, a violent commotion was seen at the base of the cliff, and the whole mass slid majestically down, forming an immense débris at the bottom. Tremendous cheers followed the blast, and a royal salute was fired.
The remarks of different intelligent observers, as to the effects of this explosion, would of course differ according to their position with respect to the scene of explosion. One observer states that “the earth trembled to the distance of half a mile; a stifled report, not loud, but deep, was heard; the base of the cliff, extending on either hand to upwards of five hundred feet, was shot as from a cannon, from under the superincumbent mass of chalk seaward; and in a few seconds not less than a million tons of chalk were dislodged by the shock, and settled gently down into the sea below.”
But the most eminent observer who has described the effects of this explosion is Sir John Herschel, from whose letter to the Athenæum we gather the following particulars. His position was on the summit of the cliff, next adjoining the scene of operations, to the southward, the nearest point to which access was permitted.
Sir John Herschel was particularly struck with “the singular and almost total absence of all those tumultuous and noisy manifestations of power, which might naturally be expected to accompany the explosion of so enormous a quantity (19,000lbs.) of gunpowder.” He describes the noise which accompanied the immediate explosion as “a low murmur, lasting hardly more than half a second, and so faint, that had a companion at my elbow been speaking in an ordinary tone of voice, I doubt not it would have passed unheeded.”
The fall of the cliff, the ruins of which extended over no less than eighteen acres of the beach, to an average depth of fourteen feet, was not accompanied with any considerable noise. “The entire absence of smoke was another and not less remarkable feature of the phenomenon. Much dust, indeed, curled out at the borders of the vast rolling and undulating mass, which spread itself like a semi-fluid body, thinning out in its progress; but this subsided instantly; and of true smoke there was absolutely not a vestige. Every part of the surface was immediately and clearly seen—the prostrate flagstaff (speedily re-erected in the place of its fall)—the broken turf, which a few seconds before had been quietly growing at the summit of the cliff—and every other detail of that extensive field of ruin, were seen immediately in all their distinctness. Full in the midst of what appeared the highest part of the expanding mass, while yet in rapid motion, my attention was attracted by a tumultuous and somewhat upward-swelling motion of the earth, whence I fully expected to see burst forth a volume of pitchy smoke, and from which my present impression is, that gas, purified from carbonaceous matter in passing through innumerable fissures of cold and damp material, was still in progress of escape; but whether so or not, the remark made at the moment is sufficient to prove the absence of any impediment to distinct vision.”
The amount of tremor experienced by Sir John Herschel at the point where he was standing was so slight, that he thinks he has felt it surpassed by a heavy waggon passing along a paved street. “The impression, slight as it was, was single and brief, and must have originated with the first shock of the powder, and not from the subsequent and prolonged rush of the ruins.” We have already noticed the remark of one observer, that “the earth trembled to the distance of half a mile;” but this seems to be a mistake; the writer fancied that it must have been so, and that he should be suspected if he were to state it otherwise. It is to be regretted that people do not endeavour to describe what they see and hear, without the embellishment of the imagination.
This grand experiment was no less grand from the absence of noise, smoke, earthquake, and fragments hurled to vast distances through the air. “I have not heard of a single scattered fragment flying out as a projectile in any direction”—continues Sir John Herschel—“and altogether the whole phenomenon was totally unlike anything which, according to ordinary ideas, could have been supposed to arise from the action of gunpowder. Strange as it may seem, this contrast between the actual and the expected effects, gave to the whole scene a character rather of sublime composure than of headlong violence—of graceful ease than of struggling effort. How quietly, in short, the gigantic power employed performed its work, may be gathered from the fact, that the operators themselves who discharged the batteries were not aware that they had taken effect, but thought the whole affair a failure, until reassured by the shout which hailed its success.”