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connexion with the zinc plate it may be called the negative pole, and if a wire attached to the positive pole is brought into contact with the disc the battery is put into action. Another tin disc E, of similar size to that of the negative pole, must slide freely along the round crossbar: the central aperture in this disc should be about an inch and a half in diameter, and to it a tin pipe, of the same diameter and about 2 inches in length, is fixed, (this is to be on one side only of the disc and must project towards the positive pole of the battery.) The use of the pipe is to keep the moveable disc steady during its motion along the bar: it must not project on both sides of the moveable disc, because it is necessary that the surfaces of the discs be in perfect contact with each other when the battery is in action.

Suppose then, a wire connected with the positive pole to be fastened to the moveable disc, and this disc to be then moved along the cross-bar until it comes in contact with the fixed disc, the battery is in action, because there is a metallic communication from one pole to the other. To enable an operator standing at a distance from the battery to bring these discs into contact, a string S, is fastened to the moveable disc and reeved through two holes bored in the fixed disc, and the two ends tied together about a foot behind the fixed disc, so as to form a span or double cord proceeding from the moveable disc. To this double cord is tied a string of any convenient length, which Mr. Roberts calls the lanyard; the end of this lanyard is carried to some place where the operator may be in safety, and on pulling it, the moveable disc slides along the cross-bar into close contact with the fixed disc, and the battery is thus put into action.

A further contrivance must however be provided to prevent the discs touching each other before the lanyard is pulled, and this Mr. R. effects by fastening to the pipe of the sliding disc a spiral wire, F, (such as a bell spring), which encircles the cross-bar, and has one end fixed to the wooden upright on the positive side of the battery. The length of the spring when unextended is such, that the moveable disc attached to it stands about 7 inches from the fixed disc, but when the lanyard is pulled, the spring extends, and the discs come in contact with each

other, and if the lanyard be slackened, the spring separates the two discs, the action of the battery is stopped, and there is then no danger in approaching the charge of gunpowder, should it not have exploded when the electricity circulated through the fine wire.

To prevent the possibility of the dises touching each other before the workmen are prepared for the explosion, a wooden pin p, is placed between them; this is inserted into a hole bored through the cross-bar about midway between the discs. When all is prepared for firing the charge, this pin, which Mr. Roberts calls the safety pin, is removed from the hole before the lanyard is pulled.

We come now to the method of connecting the long-conducting wire with the battery.

In the practice of blasting, the conducting wire should be of copper, and about one-eighth of an inch in diameter; its length must of course vary according to circumstances; but, in general, if the battery be placed 20 or 30 yards from the rock to be blown up, it will be in perfect safety.

If the battery is to be 30 yards from the explosion, 60 yards of stout copper wire covered with cotton thread well waxed will be required. The 60 yards of wire are cut in half, and the two lengths of 30 yards each laid side by side, and bound together with twine nearly in the same manner as each wire is covered with cotton. As an additional security, this double wire may be done over with sealing-wax varnish. About a foot in length of the ends of these wires is left free, that is, not bound together in the manner the rest of their length has been. (See fig. 2.)

We have now a kind of rope 30 yards long, consisting of two wires bound together, and the four ends projecting. Take one end of this rope, and fasten its two projecting ends to the galvanic battery in the following manner: Solder one projecting end to the sliding tin disc, and the other projecting end (of the same extremity of the rope) to the wire attached to the positive pole of the battery. The wire rope must be permanently fixed in this manner; in fact, it thus forms a part of the battery. When not extended for use, it may be coiled up, or wound upon a reel. Let us now suppose the wire rope extended, two of its projecting ends fixed

ROBERTS'S GALVANIC BLASTING APPARATUS.

to the battery, and let the other ends be connected by a few inches of fine wire; let us also suppose the plates immersed in the exciting liquid: if the lanyard be pulled, the sliding disc will move forward into contact with the fixed disc or negative pole, the electricity circulates from the positive pole through one part of the conducting wire, then through the fine wire (fusing it) back through the other part of the conducting wire to the sliding disc, and from this to the fixed disc, which is the negative pole of the battery.

The wire immersed in the gunpowder is generally of steel, and very fine, (of the sort called by watch-makers, balance wire) because the degree of heat raised in metals by electricity is in proportion to the minuteness of their diameter. A reel of this fine steel wire containing six or eight yards, costs 3d., and will perhaps serve for a hundred explosions. A very short piece of it is employed at one time, because the obstruction to the progress of electricity through a conductor is in proportion to its length, and, if the fine wire be too long, it will prevent the passage of electricity in sufficient quantity to fuse it.

But as it would be inconvenient to have at every explosion the trouble of attaching a fine wire to the ends of the conducting wire, Mr. Roberts has contrived a cartridge, a number of which may be kept ready for use, and one fastened without loss of time to the conducting wire whenever required.

The

cartridge is a tin tube filled with gunpowder, and in this are placed the ends of two stout copper wires connected by a fine steel wire; the copper wires are each about 10 feet long, and serve to convey the electricity from the conducting wire of the battery to the fine wire immersed in the powder of the cartridge. These copper wires he calls communicating wires. The tube is stopped at both ends by corks covered with cement to keep the gunpowder dry; when thus corked and cemented the cartridge may be fired under water without a risk of failure. The communicating wires must be of sufficient length to extend from the bottom of the bore-hole in the rock to a few feet above the surface, and as the holes are seldom more than 6 or 8 feet deep, 10 feet may be taken as the average length of the communicating wires.

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The details of making the cartridges are as follow:

:

"Take 20 feet of stout copper wire covered with cotton thread, double it, and twist the two parts at the looped end closely together for about 6 inches of their length, A, fig. 3; then, with a file or cutting pliers, cut off the round end of the loop, and the ends will project as horns of half an inch in length BB; then bare the extreme points of these horns (being about half an inch asunder) of the cotton thread that is around them, and clean them with a file now take half an inch of the fine steel wire, lay it across from horn to horn of the stout wire, and there (C) let it be firmly soldered. We have now two long stout copper wires connected at one extremity by a fine steel wire: the end of these wires are twisted together to prevent the horns slipping into contact with each other, and also to preserve the fine wire from being broken by any pull or jerk given to one of the communicating wires. As this combination of wires is placed in the bore-hole, it will be exposed to the action of the ignited charge of gunpowder, and, without some precaution to secure it, would be destroyed by every explosion: to prevent this waste, the communicating wire is first covered with cord (in the same manner as the conducting wire of the battery is covered), and an additional covering is then given of hard whip-cord or of fine binding-wire, (binding-wire will perhaps be found best, as it effectually prevents the included communicating wires being injured by the broken fragments of the rock). The fine wire soldered to the ends of the communicating wire will be destroyed at each discharge, for the electricity will fuse it, but this fine wire is easily replaced at a cost of three-halfpence for every dozen cartridges.

"The body of the cartridge is a tin tube, 3 inches long, and of an inch or an inch in diameter, of which the joint is soldered and rendered perfectly water-tight: the fine wire across the horns of the twisted wire is placed in the centre of the tube, and retained firmly in this position by a cork at the end of the tube, through which the twisted wires pass, fig. 4. The best way of fixing the twisted wires is to split a bit of cork half through, lay them in the slit, then force the cork into the tube, and this will jam the wires firmly in the slit: taking care that the horns do not touch the sides of the cartridge, and that the cork is covered with a good cement, as this assists in preserving the horns in their proper position. The cement I generally use is composed of one part of bees'-wax and two of resin, which, if put on hot, readily sets, is very strong, and does not crack in cooling;

but any cement that has these properties, and effectually keeps out damp, will answer the purpose. Having now the tin tube with the fine wire firmly fixed in the centre, the next operation is to fill the cartridge with gunpowder. It must be fine sporting powder and thoroughly dry; unless this be attended to, the fine wire may be fused by the electric fluid without igniting the charge, for the action is so rapid, that if the powder be damp, it will hardly be dried, much less ignited by the fusion of the wire. The best method of ensuring this dryness, when a great number of cartridges are made at one time, is to dry the powder over a steam-tight box filled with boiling water; but, when a few dozen only of cartridges are required, heat a soup plate by a fire, and when it is a little hotter than the hand can bear, take the plate from the fire, and throw into the plate a sufficient quantity of powder to fill two or three cartridges; shake it in the hot plate for two or three minutes, and then fill the cartridge tubes with the powder, which will now be perfectly dry and warm-while in this state cork the ends of the cartridges, and cover the corks with the same kind of cement as that used for the corks through which the wire passes."

It only remains now to detail the actual process of blasting with the apparatus before described.

When a rock is to be rent by the explosive force of gunpowder, the first thing done is to bore in the rock a hole, of a depth and diameter proportioned to the strength of the stone and the quantity we wish detached. Let us, for example, suppose the hole to be 6 feet deep and 2 inches in diameter: cleanse it from dust and moisture by passing a straw or oakum wad several times through it, then lightly pour into the hole half the intended charge of gunpowder; put a cartridge upon this, and upon the cartridge pour the remainder of the charge; do not ram the powder down, for the lighter it lies together the better: the cartridge will thus be in the centre of the charge, and its long communicating wires will project 3 or 4 feet above the surface of the rock: the charge of powder and cartridge will fill about 8 or 10 inches of the hole.

The next operation is tamping. Thrust a straw or oakum wad gently down the bore-hole until it is about 24 feet from the surface; this done, there remains an empty space (that is to say, containing merely atmospheric air,) of about 24 feet in depth between the wad and the gun

powder. In practice Mr. Roberts has found it of great importance to allow this distance to exist between the powder and the wad, for the expansion of the air by the flame of the ignited powder adds to the rending force, and there is also an effect produced similar to that when a ball is rammed but half-way down a musket barrel. When the wad is in its proper place fill the hole up to the surface of the rock with dry sand. The hole is now charged, and about 4 feet of the cartridge communicating wires project above the surface of the rock.

Having filled the box of the battery with a saturated solution of sulphate of copper mixed with a little sulphuric acid, place it at some convenient distance from the rock, behind a large stone, or in any situation where it is not likely to receive injury from the falling fragments of the rock; put the frame of plates on the ground by the side of the box, and be careful the safety pin p, is in the hole prepared for it; then unroll the conducting wire, and attach the ends that are free to the cartridge communicating wires projecting above the surface of the rock. This attachment may be done by twisting them together, but it will be better that a binding-screw, fig. 5, be soldered to each free end of the conducting wire, and to these the communicating wires are readily attached by inserting an end into each screw, two or three turns of which will make the contact perfect. Fig. 5, represents the screw; b, hole for insertion of the communicating wire; c, the conducting wire soldered to the bindingscrew, and d, the screw. This contrivance will be found of great service, because the cartridge can be attached to the conducting wire without loss of time, a good metallic contact between them is ensured, and, if the binding-screws are covered with cotton, varnish, or some other insulating substance, there will be no metallic contact between the separate parts of the conducting wire; and this should be avoided, because it would open a channel to divert the electric fluid from its proper course.

When the cartridge has been fastened to the conducting wire of the battery, unroll the lanyard, and carry the end to a situation where the operator can stand in perfect safety. Every one must now retire from the rock, except one person, whose office will be to ascertain that the

ADULTERATION OF ZINC.

safety-pin is in its place, and that the discs do not touch each other; he is then to place the box in such a position, that its end shall be towards the point from which the lanyard is pulled, taking care this and the double cord are clear; he then puts the frame of plates into the box,-a pair of plates into each cell, being careful the fixed disc is towards the place where he stands to pull the lanyard: the safety-pin must now be taken out, and the operator retires to the place where the lanyard has already been laid; he then pulls the lanyard slowly and steadily, without a jerking motion; the moveable disc slides into contact with the fixed disc, the electricity circulates, and the charge of powder is exploded. Fig. 6 shows all the apparatus in proper order for firing.

In this figure a a are the communicating wires of cartridge. c. Cartridge.

p. Powder.

w. Straw or oakum wad.

bb. Binding screw. xx. Conducting wire. 7. Lanyard.

m. Man to pull the lanyard. d. Battery.

s. Vacant space.

t. Tamping stuff.

The operator must after the explosion return to the battery, and remove the plates from the cells, coil up the lanyard, detach the conducting wire from the cartridge communicating wires, and coil it up. The communicating wires, most probably, will be found jammed between the fragments of the rock, and there they must remain until released by carrying away the stones, when the wires will be found uninjured :-if they are forcibly pulled out from the fragments of rock, they may be broken: the tin tube and fine wire of the cartridge will be destroyed by the force of the explosion, but the communicating wires will serve for another cartridge. It sometimes happens that, if the battery has not been used for some days, the papers that are round the zinc become so dry, that time is required for the exciting solution to penetrate through them to the zinc, and if an attempt be made to work the battery before the paper is well saturated with the liquid, it is probable no electricity will circulate to avoid such a disappointment, either dip the frame of plates into

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a tub of water, for the space of five or ten minutes, or allow the plates to remain for a few minutes in the battery cells before the lanyard is pulled.

Mr. Roberts gives, also, a description of a method by which several charges may be fired simultaneously, which is stated to have been found of great service at Skerry Vore Lighthouse, now erecting under the superintendance of Mr. Allan Stevenson; but for this, as for many other illustrative details, we must once more refer to the pamphlet itself, which does altogether great credit to the scientific sagacity and practical skill of its author.

ADULTERATION OF ZINC.

Sir, I beg to direct public notice, through your pages, to an important fraud in the market of metals, which I have recently discovered. Having had occasion for a considerable quantity of zinc, in as great a state of purity as I could readily obtain it, for the purposes of my patented method of preventing the corrosion of iron, I purchased and employed a quantity of waste clippings of sheet or patent zinc, obtained from a zinc-worker's establishment, and stated to consist principally of the best Belgian zinc. Circumstances occurred, during its use, to make me suspect this zinc contained some foreign metal; and on submitting to analysis several different specimens of the sheet zinc, just as received, I found them all to consist of variable mixtures of lead and zinc. The alloy of lead in some amounted to nearly one-fourth the amount of the zinc; in one specimen, to one-third; and in the greater number to between a sixth and a fifth of the weight of zinc. My curiosity having been excited, I have since examined several other specimens of sheet zinc, and find most of them alloyed with lead. This adulteration is obviously of the most mischievous tendency, as regards the chief purposes to which sheet zinc is applied, promoting its oxidation, and increasing its weight as a covering.

The end in view in the adulteration is, however, abundantly plain. Zinc is now about 491. per ton, in sheets; lead is only 197. per ton: hence, a metal consisting of three parts zinc, and one part lead-crude zinc being 371. per ton-will

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"Those who have visited the exhibition held some time since at the Mechanics' Institution, will probably recollect a small model railway in the room appropriated to illustrations of hydrostatics, in which a tiny iron carriage was made to run down an inclined plane, traverse an iron circle, and ascend an inclined plane on the opposite side. The curiosity attracted much attention, and those who did not fully comprehend the principle of its operation were not a little puzzled at the wonderful precision with which the little vehicle performed its rapid journey. If the miniature railway was deemed wonderful, we know not what the public will now think of that exhibiting at the theatre. We had the pleasure of viewing it in its full operation on Friday last. A carriage, sufficiently large to hold

a man or a woman, is loaded with heavy weights, and despatched down a plane which reaches from the upper portion of the gallery down to the orchestra; here, by its own impulse, it traverses a vertical circle of 40 feet diameter, and the remaining force is expended in carrying it forward up another slope, which reaches to the back of the stage. The journey having been performed with fifty-six pound weights, a bucket of water, &c., without either of the weights or a drop of liquid having been displaced; a young man, and subsequently a young lady, entered the car, and each performed the apparently perilous journey in perfect safety, and without the slightest inconvenience. The exhibition is exceedingly curious, and well worthy the attention of the public."

The invention noticed in the preceding extract, is the same, we believe, which our readers will find included in our List of Registered Designs for the last month, No. 1196, April 14, Messrs. Hutchinson, Higgins and others, proprietors. The principle of the thing is well known, though the application of it to enabling people to travel with their heels uppermost is, doubtless, new. The prefixed engravings are copies on a reduced scale of those deposited at the Registration Office,

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