These are little hollow cups of copper having a fulminating substance at the bottom, so that when put on to the “nipple” of the gun and struck by the “hammer,” the fulminating powder explodes, and the spark passing down the hole in the nipple discharges the gun. To prepare the fulminating powder for these caps, let 100 grains of mercury be dissolved in a measured ounce-and-a-half of nitric acid, and when cold let two ounces of spirits of wine be added, and the whole put into a Florence-oil flask made perfectly clean, and let it be placed in the open air; copious fumes will pass off and a violent action take place, during which a white crystalline powder will be deposited; as soon as all action has ceased and the liquid cooled, pour the whole on a filter of blotting paper, and let the fluid pass through, wash the powder which remains on the filter with a little water, and let it dry, without heat. This is fulminating mercury, which is a highly dangerous compound, and should be kept in a bottle with a cork, and not a stopper, as the friction of this against the neck of the bottle might cause an explosion.At the Arsenal at Woolwich is a machine (shown at the head of this article) which makes the caps complete. It is fed by a band of thin copper about two inches wide, out of which pieces are punched in the form of a thick cross, leaving the perforated copper as shown in [fig. 1]; these pieces are punched or “struck up,” and expelled as perfect caps of the form of [fig. 2], at the rate of about 1000 per minute. The caps have next to receive their charge of fulminating powder, which is done by dropping them into a perforated plate ([fig. 3]), capable of receiving many hundreds; this is covered over by two other plates, each perforated to correspond, but the upper one made to shift, so that in one position the holes correspond, and in the other they do not, but remain as small shallow dints. A portion of the fulminating powder is put on this plate and scraped all over it by means of a piece of paste-board ([fig. 4]), so as to fill all the little dints; the plate is then shifted and the holes made to correspond, when the powder falls through into the percussion caps, each one of which thus receives a definite charge. The next process is to press it down into the cap, so as to prevent it falling out. For this purpose the plate full of caps is removed to a machine ([fig. 5]), having a row of little stoppers, which are moved rapidly up and down, the pressure being exactly regulated by flat leaden weights, suspended so as to give only so much pressure as will consolidate the powder, without exploding it. They are next removed to a third machine called the “varnishing machine” ([fig. 6]). This has a trough of varnish, made by dissolving shellac in spirits of wine, into which a row of wires dip, and by a turn of the hand convey the minute portion of varnish on their points into the caps, row after row. This varnish dries in a few minutes, and causes the fulminating powder to adhere; the caps are now complete. The old form of caps was a simple short cylinder, but it is found better to allow the four little flaps to remain on, that the right end may be distinguished and instantly placed on the nipple the proper side downwards, which cannot otherwise so readily be done in the dark, and when the hands are benumbed with cold.
PUMPS AND FIRE ENGINES.
FIG. 1.
FIG. 2.
Pumps are used for lifting fluids above their level into some higher situation, such as from the hold of a ship or from a well. [Fig. 1] shows the different parts of a common “lifting pump;” a a is a cylinder, b a piston rod or “plunge,” c the sucker made of leather to fit nicely the cylinder, d a valve in the sucker to open upwards, e a valve fixed to the cylinder, also to open upwards, f a box with a spout.
The piston being raised by lowering the handle of the pump, a partial vacuum is formed below the upper valve, which shuts down directly the piston is raised by the pressure of the air; this vacuum causes the external air to force the water some way up the tube g. On the piston descending, the lower valve is forced down and the upper one opened, this keeps the water where it is and allows the piston to descend without forcing the water down again, and on its being raised a second time the upper valve shuts and the lower one opens, the water being drawn up still higher, and this takes place till the box at the top is full to the spout, when it runs out. The air-pump is on the same principle, and is generally made with two cylinders worked by means of a “rack” and wheel ([fig. 2]); this is only to save time, instead of pumping water it pumps out air from any vessel called a “receiver,” because it receives any object to be placed in a “vacuum,” that is to say a partial vacuum, for the air-pump cannot produce a vacuum, as the air is only partly removed by each stroke of the piston, leaving the air more rarefied inside; and although each stroke of the piston increases the rarefaction, yet it cannot get all, as it merely takes part, and always leaves part.
