But with the firearm a vast volume of expansible gas is kept locked up in the form of gunpowder, gun-cotton, fulminating silver, or other explosive compound, and is let loose, when wanted, by the aid of fire. * * * * * [Illustration: CHŒTODON, OR ARCHER-FISH.] [Illustration: BLOW-GUNS--CANNON.] In the illustration are represented on the right hand the blow-guns of America and Borneo, and below them is the cannon as at present made. On the left hand of the same illustration is seen a representation of a natural gun which has existed for thousands of years before gunpowder was invented, and very long before the savage of Borneo or America discovered the blow-gun. It is the ARCHER-FISH (_Chœtodon_), which possesses the curious power of feeding itself by shooting drops of water at flies, and very seldom failing to secure its prey. There are several species of this very curious fish spread over the warmer parts of the world, and their remarkable mode of obtaining prey is very well known in all. There is, indeed, scarcely any phenomenon in Nature more remarkable than the fact of a fish being able to shoot a fly with a drop of water projected through its tubular beak, if we may use that expression for so curiously modified a mouth. Indeed, so certain is the fish of its aim, that in Japan it is kept as a pet in glass vases, just as we keep gold fish in England, and is fed by holding flies or other insects to it on the end of a rod a few inches above the surface of the water. The fish is sure to see the insect, and equally sure to bring it down with a drop of water propelled through its beak. It is worthy of remark that the same principle was once, though unsuccessfully, employed in the propulsion of carriages, under the name of the Pneumatic Railway. Some of my readers may remember the railway itself, or at all events the disused tubes which lay for so many years along the Croydon Railway. Speed was obtained, as I can testify from personal experience, but the expense of air-pumps and air-tight tubing was too great to be covered by the income, especially as the rats ate the oiled leather which covered the valves. * * * * * I FIND some little difficulty in arranging the subject which comes next in order. It might very properly be ranked among the Levers, which will be treated of in another chapter; or it might be placed among the examples of centrifugal force, together with the sling, the “governor” of the steam-engine, &c., all of which will be more fully described in their places. However, as we are on the subject of Projectiles, we may as well take it in the present place.