Page:The Development of Navies During the Last Half-Century.djvu/236

 escape the vessel is lightened to a corresponding extent. Consequently a torpedo propelled by this motive power would, as the air escaped after doing its work, be continually getting lighter, and if started 10 ft. under water would soon come to the surface and finish its course in this position. This had to be corrected and provided for by special machinery. The torpedo must travel throughout its run in the same horizontal plane. The method by which Mr Whitehead obtains this effectually is the most ingenious part of his invention. It was long kept a secret, but has recently been made public. The general principle is to utilise the pressures due to different depths of water to actuate horizontal rudders, so that the torpedo is steered upwards or downwards as its tendency is to sink or rise.

In the foremost end of the torpedo the explosive is carried, originally gunpowder but now gun cotton, as being so much more powerful, its ignition being effected by the impact of the nose of the torpedo on any hard substance, which forces a pointed striker against a detonating cap.

So constructed, we have an iron or steel fish-shaped body, propelled by air highly compressed, which can be regulated to give a high speed for a short distance or a greater range at a lower rate. By mechanical contrivances the torpedo could be set to run any required distance, and then come to the surface or sink. The firing arrangement was not operative until it had proceeded some way on its course, but for exercise it was necessary that this should be in a condition of safety when