At the head of the _Whitehead_ is the end of a pointed steel rod which penetrates the chamber containing the explosive. When the torpedo’s nose comes into contact with a ship’s side, or in fact any rigid object, the point of this rod is driven in against a detonator cap inserted in the centre of the charge: the immediate result is an explosion sufficient to tear a large hole in the ship’s hull. The detonator is fulminate of mercury, which, when ignited by a sudden blow, expands to about 2,500 times its own size. The sudden expansion gives such a severe blow to the gun-cotton around it that it at once explodes. Special precautions have to be taken to prevent the torpedo from damaging the ship from which it is fired: it might happen through carelessness that a lieutenant would fire one with the port closed, and so three checks are provided. The rod is so arranged that it cannot go back until a small “collar” with propeller fans on it has revolved off. When the torpedo enters the water the fans begin to turn, and when it has run some 30 yards the collar is worked off. Even then the charge will not explode unless the blow to the rod is severe enough to shear off a little copper pin standing in the way. Lastly there is a third precaution in the shape of a safety pin which holds the collar fixed until it is withdrawn at the last moment as the torpedo is launched into the tube. It happened in the Russo-Turkish war that a Russian lieutenant in command of a torpedo-boat forgot to haul out the “safety pin” and the consequence was that though the torpedo reached the target it failed to explode. From what has been said it will be understood that torpedo warfare is not quite so simple as it looks. In time of peace the torpedo is not fitted with its war head, and so for daily purposes a steel dummy head is used, while there is an arrangement that causes it to rise to the surface on completion of their run. To facilitate its recovery, a “Holme’s light” is carried on to the head. This consists of an arrow-headed tin canister pierced with tubes and full of phosphide of calcium, which on contact with the water gives out both a strong light and a strong smell. 3. THE AIR CHAMBER. This contains the motive power of the torpedo and it comes just behind the explosive chamber. The air is compressed into the compartment by means of air-compressing pumps fitted on board ship, and the latest types are tested to a pressure of 1,700 lbs. to the square inch. 4. THE BALANCE CHAMBER.