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A joint of bamboo, see fig. 9 b, about 5 inches long by 1 in diameter, capable of holding 2lbs. of mercury, was fitted with a small brass stopcock below, through which when opened the mercury might escape. It was found by experiment, that when fully open 2lbs. escaped through the stopcock employed in a few seconds more than five minutes.
The bamboo joint was fixed on a wooden frame ƒƒ, having a vessel below to receive the mercury. A stout copper wire was led through the diameter of the bamboo, one and a half inch from the bottom. A similar wire was inserted three-fourths of an inch below, and in the same direction with the first, and this second wire was divided into two parts, as shewn in the drawing. The ends of the wires were turned into a few loose spirals to allow of their being readily connected with the battery on one side, and with the conductors to the mine on the other.
Suppose this joint filled with mercury, the stopcock shut, and the battery wires connected with it at one side (say the right,) and the mine conductors connected with it at the left-in this case metallic contact being established in the conductors c, c, between the battery and the mine, no ignition can possibly occur, because the electricity returns to the battery by the first cross road it meets, if I may be permitted to use this homely, but I think expressive, illustration.
If we now open the stopcock and allow the mercury to trickle out as soon as its level subsides below that of the highest copper wire, the only path of the electric fluid now lies through the mine, the platinum becomes ignited, and explosion ensues.
But should any accident have occurred, so that no explosion takes place at once, and should therefore the whole arrangement need inspection, the mercury still subsiding passes after two or three minutes below the second wire, which having been previously cut, the circuit is now completely interrupted, and the whole arrangement is perfectly safe for inspection.
In using this apparatus two things must be attentively borne in mind. No accident can happen while it is full of mercury, but when once emptied it must not be filled again while in connexion with the battery, otherwise an explosion may ensue.
The whole arrangement is shewn in the accompanying diagram, in
which a represents the battery, b the bamboo mercury cup nearly full of mercury, c c the battery conductors, c c the conductors leading to the explosion tube t, containing the platinum wire and priming.
The whole cost of this apparatus, including quicksilver, is not more than six rupees.
It is obvious that many substitutes for the watch apparatus may be devised, and indeed the expense of even the cheapest watch procurable (ten rupees) is an objection, though an insignificant one, to its employment, where circumstances render it advisable to expend all the apparatus by placing it immediately over the mine. In a remarkable set of experiments which I witnessed, portfires were employed instead of the watch-one, six inches in length, supported by a string the wire for establishing the connexion with the battery, corresponding to the watch wires a, a—a second portfire, nine inches long, supported a weight, the descent of which was intended to break the connexion in the manner effected by the watch wires b, b. In two of the three trials in question the weight did not fall, and the consequence was the imminent danger of the destruction of the party whose duty it was to re-examine and re-adjust the arrangements on the failure of the two first attempts. In one of the trials it was observed too, that one of the tubes containing the mercury was completely choked up by melted saltpetre which had fallen from the portfire during its combustion. These defects seem to me to constitute a most serious objection to the use of portfires; I admit, however, that these are more of a military character than my contrivances-and in this, I believe, their chief merit lies.
Explosion of the barque "Equitable."
The barque "Equitable," bound to Sydney, and laden with wheat, rice, rum, &c. while proceeding down the river in September 1839, touched on Fultah Sand, and instantly turned over in six to seven fathoms water. The wreck lay on her beam-ends athwart Fultah Channel, the keel towards Calcutta. On sounding with the lead, the water over her quarter shoaled to three fathoms, and then suddenly deepened to five or six.
Capt. Fitzgerald, the engineer officer employed, determined to attempt the destruction of the vessel by the explosion of 2,500 lbs. of powder placed between the mizen and main masts, close to the deck.
The cylinder already described was admirably fitted up, under Captain Fitzgerald's directions, in the arsenal of Fort William; before being filled with powder, the exploding tube was screwed into its side, twenty-four barrels of powder were then poured in through an aperture left at the top of the cylinder, which was afterwards closed with wood and soldered up with sheet lead.
The cylinder thus prepared was slung on a cradle to the bows of the "Vulcan" anchor vessel, which proceeded down the river and took up her berth at Fultah, immediately over the wreck.
At the slack of the tide, on the 14th December, the preparations for lowering the cylinder being completed, the ends of the ladder-conductor were securely twisted to the wires projecting from the explosion-tube, a piece of wood interposed, and the whole guarded by a joint of bamboo and a wedge. As the cylinder was lowered, my assistant, Mr. Siddons, cautiously permitted the ladder conductors to follow, and when the cylinder was in its berth, the conductors were cut short, so as that their free ends should reach the bow of an old fishing boat, previously moored fore and aft over the wreck. To the bow of this boat the wires were secured by twisting them round screws inserted for the purpose; the length of conductors immersed in the water was thirty-four feet.
The battery and watch apparatus were placed on the boat-the watch set to twelve minutes-and, lastly, the battery wires twisted to the conductors at the bow. The party at the mine consisted of Capts. Fitzgerald and Debude, and Lieut. Smith of the Engineers, my assistant Mr. Siddons, and myself. When all was ready, one of the wires in the battery, purposely left out of its mercury cell to prevent accident, was placed in its position, and our party pulled away vigorously from the dangerous vicinity. At the thirteenth minute a slight concussion was felt in our boat, a sound like that of a very distant and heavy gun at sea was heard, and a huge hemispherical mass of discoloured water was thrown up to the height of about 30 feet. From the centre of this mass there then rose slowly and majestically a pillar of water, intermingled with foam and fragments of wreck, and preserving a cylindrical form till it reached an elevation of at least 150 feet. The column then subsided slowly, a wreath of foam and sparkling jets of water following its descent, and rendering the spectacle one of indescribable beauty.
On pulling to the spot we found the river absolutely thickened by the wreck and cargo of the vessel. By subsequent examination, it was found that with the exception of the forecastle, the "Equitable" had by this explosion been literally torn to pieces. The fishing boat, battery, watch, &c. were all "expended."-The ladder conductors were however picked up uninjured half a mile from the wreck.
In conclusion of this paper-which circumstances induce me to publish sooner than I intended--I think it but just to express my thanks for the zealous assistance afforded me in all the preceding experiments, and in the construction of the apparatus, by Mr. Siddons, of the Medical College.
I should add, that while my experiments were proceeding, my colleague Mr. Egerton, suggested the placing of a strip of saltpetre matchpaper round the platinum, in preference to placing this in contact with the powder. The excellence of the suggestion was proved by experiment, for we found on repeated trials that saltpetre match inflames at nearly double the distance at which the wire will explode powder.
The match should be prepared by immersing cotton in a saturated solution of the purest saltpetre; if the salt be impure the match is liable to become damp, and thus to frustrate the experiment. A few fibres of this cotton should be twisted loosely round the platinum wire.
It is astonishing to observe the great cooling effect produced on the platinum wire by the contact of apparently dry powder, if this be in the least degree damp. On one occasion with a new battery in perfect order, with dry conductors only 150 feet long, the standard platinum wire was kept for an hour in a pint bottle of powder just drawn from the canister, and no explosion ensued; but by removing some of the powder, so as to leave only a grain or two on the wire, the mass being half an inch below it in the neck of the bottle, explosion took place the instant the battery contact was effected.
I wish it to be remembered too that the preceding experiments are applicable only to explosions in fresh water; operations in salt water would require a special set of experiments, which I have not had the means of instituting on a sufficiently large scale. I have strong reason however to believe that the cork conductors with pitched wires will