TIME GUN.
Time guns.
One use to which one of our guns was put is shown in the illustration. We were asked to repair the clock, but this is always difficult, and it is uncertain how long it may go correctly afterwards. We therefore erected a frame over the gun, and fitted the lens of a camera on an axle placed due east and west, so that it could turn in the plane of the meridian, and so be adapted to the sun’s gradual change of declination. Below the lens we fitted a piece of tin with its edges turned downwards, to hold a piece of quick match, a strip of calico, steeped in a strong solution of gunpowder, beneath it; a small slit in the tin was then so adjusted as to let the focus of light fall through it exactly at 12 o’clock; a small clip of tin confined the other end of the match over the vent. The moment of noon was announced with a regularity that no clock in our possession could have attained; and one great advantage was, that if by the interposition of a cloud, which would not happen once in nine months, the gun should fail to fire at the proper moment, it could not go wrong, for the speck of light would pass the narrow slit, and no discharge would take place till the next day.
The absence of the cap squares of a gun can be remedied by lashing the metal firmly down to the carriage with a raw hide rope, and then twisting it up tight with a stick, as shown in the above illustration; which also exhibits the mode of raising a gun by making use of the trail as a lever. A heavy gun may be mounted by letting its muzzle into a hole in the ground while the carriage is run under it.
Percussion caps and substitutes.
During the continuance of the Damara and Hottentot war we were becoming exceedingly short of percussion caps, and were obliged not only very carefully to husband the few that were left, but to turn our attention to the manufacture of substitutes. The tips of Congreve matches, with the wood cut to a point so as to stick in the nipple of the gun, proved very effective, but were liable to be brushed or shaken off, or to become damp if carried for any length of time before firing. We, therefore, first inclosed the end of the match in the shell of an expended cap, and finding this answer, we dissolved the composition, and put a drop into the cap without the wood; we then dissolved it off a whole box of matches at a time, and with a camel-hair pencil put small drops into as many cap shells as it would serve. This answered admirably; but our next fear was that the supply of matches would run short, and therefore, drawing on our own artificial horizon for the quicksilver, on the photographic stores for nitric acid, and on our friends, the missionaries, for a supply of alcohol from their natural-history department, we set about the manufacture of fulminate of mercury according to the following recipe:—Dissolve 10 grs. of mercury in 1½ oz., by measure, of nitric acid; the solution is poured cold into 2 oz., by measure, of alcohol in a glass vessel, and gentle heat is applied till effervescence is excited, though it ordinarily comes on at common temperatures, a white vapour undulates on the surface, and a powder is gradually precipitated, which is immediately to be collected in a filter, well washed, and cautiously dried. It detonates by gentle heat or slight friction. Two grains and a half, with one-sixth of gunpowder, form the quantity for one percussion cap. We used a conical twist of blotting-paper for the filter, and mixed the fulminate, while still moist, with a small palette knife upon a plate with the gunpowder, treating it very gently, and in small quantities. We collected all the shells of expended caps, and for new ones cut out a cross of thin copper; then, making a hole in a piece of iron and a punch the size of the nipple, we drove the centre of the cross in, and the shell was formed. Stiff cartridge-paper stiffened with gum would have answered for dry weather, but would not have been secure against wet.