The engines consist of three single-action cylinders, their cranks working at an angle of 120° to one another, so that there is no “dead” or stopping point in their action. They are very small, but, thanks to the huge pressure in the air chamber, develop nearly thirty-one horse-power. Lest they should “race,” or revolve too quickly, while passing from the tube to the water and do themselves serious damage, they are provided with a “delay action valve,” which is opened by the impact of the torpedo against the water. Further, lest the air should be admitted to the cylinders at a very high pressure gradually decreasing to zero, a “reducing valve” or governor is added to keep the engines running at a constant speed. Whitehead torpedoes are fired from tubes above or below the waterline. Deck tubes have the advantage of being more easily aimed, but when loaded they are a source of danger, as any stray bullet or shell from an enemy’s ship might explode the torpedo with dire results. There is therefore an increasing preference for submerged tubes. An ingenious device is used for aiming the torpedo, which makes allowances for the speed of the ship from which it is fired, the speed of the ship aimed at, and the speed of the torpedo itself. When the moment for firing arrives, the officer in charge presses an electric button, which sets in motion an electric magnet fixed to the side of the tube. The magnet releases a heavy ball which falls and turns the “firing rod.” Compressed air or a powder discharge is brought to bear on the rear end of the torpedo, which, if submerged, darts out from the vessel’s side along a guiding bar, from which it is released at both ends simultaneously, thus avoiding the great deflection towards the stern which would occur were a broadside torpedo not held at the nose till the tail is clear. This guiding apparatus enables a torpedo to leave the side of a vessel travelling at high speed almost at right angles to the vessel’s path. It will be easily understood that a Whitehead torpedo is a costly projectile, and that its value--£500 or more--makes the authorities very careful of its welfare. During practice with “blank” torpedoes a “Holmes light” is attached. This light is a canister full of calcium phosphide to which water penetrates through numerous holes, causing gas to be thrown off and rise to the surface, where, on meeting with the oxygen of the air, it bursts into flame and gives off dense volumes of heavy smoke, disclosing the position of the torpedo by night or day.