The Brennan is used by our naval authorities. It is the invention of a Melbourne watchmaker. Being a comparatively poor man, Mr. Brennan applied to the Colonial Government for grants to aid him in the manufacture and development of his torpedo, and he was supplied with sufficient money to perfect it. In 1881 he was requested by our Admiralty to bring his invention to England, where it was experimented upon, and pronounced so efficient for harbour and creek defence that at the advice of the Royal Engineers Mr. Brennan was paid large sums for his patents and services. The Brennan torpedo derives its motive power from a very powerful engine on shore, capable of developing 100 horse-power, with which it is connected by stout piano wires. One end of these wires is wound on two reels inside the torpedo, each working a screw; the other end is attached to two winding drums driven at high velocity by the engine on shore. As the drums wind in the wire the reels in the torpedo revolve; consequently, the harder the torpedo is pulled back the faster it moves forward, liked a trained trotting mare. The steering of the torpedo is effected by alterations in the relative speeds of the drums, and consequently of the screws. The drums run loose on the engine axle, and are thrown in or out of gear by means of a friction-brake, so that their speed can be regulated without altering the pace of the engines. Any increase in the speed of one drum causes a corresponding decrease in the speed of the other. The torpedo can be steered easily to right or left within an arc of forty degrees on each side of straight ahead; but when once launched it cannot be retrieved except by means of a boat. Its path is marked by a Holmes light, described above. It has a 200-lb. gun-cotton charge, and is fitted with an apparatus for maintaining a proper depth very similar to that used in the Whitehead torpedo. The Sims-Edison torpedo differs from the Brennan in its greater obedience to orders and in its motive power being electrically transmitted through a single connecting cable. It is over thirty feet in length and two feet in diameter. Attached to the torpedo proper by rods is a large copper float, furnished with balls to show the operator the path of the torpedo. The torpedo itself is in four parts: the explosive head; the magazine of electric cables, which is paid out as the torpedo travels; the motor room; and the compartment containing the steering-gear. The projectile has a high speed and long range--over four thousand yards. It can twist and turn in any direction, and, if need be, be called to heel. Like the Brennan, it has the disadvantage of a long trailing wire, which could easily become entangled; and it might be put out of action by any damage inflicted on its float by the enemy’s guns. But it is likely to prove a very effective harbour-guard if brought to the test.