Page:The Development of Navies During the Last Half-Century.djvu/222

 similar portions are taken away, those remaining being opposite the blank spaces on the block. Consequently the latter can be pushed straight into the gun, and a portion of a turn engages the screw threads on each, so locking the breech without the loss of time involved by screwing the block in the ordinary way. As the cap or tube which ignites the charge is placed in this plug, which has a channel through the centre to allow the flame to pass into the chamber of the gun, there is a mechanical arrangement for preventing the tube being inserted until the breech piece—as this plug is termed—is thus locked. To prevent escape of gas to the rear it is necessary to effectually seal the end of the powder chamber. This is done by securing to the inner end of the breech piece either a thin steel cup, which on discharge of the gun is expanded against the inner sides of the chamber, or a pad of asbestos, which under pressure of the powder gas performs the same function. This, in technical language, is the obturator, and when we consider that the ignited gas exerts a pressure of some 15 tons to the square inch the importance of confining its energy to the base of the projectile may be understood.

As regards method of ignition, we have for some years utilised electricity for this purpose. On an unstable platform, such as a ship presents, it may be readily conceived that any delay in discharging a gun when it bears on an object must result in a miss. The remedy our ancestors had for this was to attain such close proximity to their object that a certain proportion