On land compressed-air installations include reservoirs of large size in which air can be stored till needed, and which take the place of the accumulator used with hydraulic power. On shipboard want of space reduces such reservoirs to minimum dimensions, so that the compressors must squirt their air almost directly into the cylinders which do the work. When the load, or work, is constantly varying, this direct drive proves somewhat of a nuisance, since the compressors, if worked continuously at their maximum capacity, must waste large quantities of air, while if run spasmodically, as occasion demands, they require much more attention. It is therefore considered advisable by some marine engineers to make compressed air perform as many functions as possible when it is present on a vessel. The United States monitor Terror is an instance of a warship which depends on this agency for working her guns and turrets, handling ammunition, and—a somewhat unusual practice—controlling the helm. The last operation is performed by two large cylinders placed face to face athwart the ship. They have a common piston-rod, in the middle of which is a slot for the tiller to pass through. Air is admitted to the cylinders by a valve which is controlled by wires passing over a train of wheels from different stations on the ship. An ingenious device automatically prevents the tiller from moving over too fast, and also helps to lessen the shocks given to the rudder by a heavy sea.
We now come to electricity, the fourth and most modern form of transmission. Its chief recommendation is that the wires through which it flows lend themselves readily to a tortuous course without in any way throttling the passage of power. And as every ship must carry a generating plant for lighting purposes, the same staff will serve to tend a second plant for auxiliary machinery. Electric motors work with practically no vibration, are light for their power, and can be very easily controlled from a distance. They therefore enjoy increasing favour; and are found in deck-winches, anchor-capstans, ammunition hoists, ventilation blowers, and cranes. They also control the movements of gun-turrets, having been found most suitable for this work.
If the current were to get loose in a ship it would undoubtedly cause more damage than an escape of compressed air or water. Electricity, even when every known means of keeping it within bounds has been tried, is suspected of causing deterioration to the metalwork of ships. But these disadvantages are not serious enough to hamper the progress of electrical science as applied to marine engineering; and the undoubted economy of the electric motor, its noiselessness, its manageableness, and comparatively small size will, no doubt, in the future lead to its much more extensive use on board our floating palaces and floating forts.
FOOTNOTES:
[16.] F. M. Bennett, in the Journal of the American Society of Naval Engineers.
[17.] Charles V. Daly, in The Magazine of Commerce.
[CHAPTER XIII]
"THE NURSE OF THE NAVY"
Just as a navy requires floating distilleries, floating coal stores and floating docks, so does it find very important uses for a floating workshop, which can accompany a fleet to sea and execute such repairs as might otherwise entail the return of a ship to port.