106 NAVAL SCIENCES. 



and positions in the ship, either on deck, in the shrouds, in the tops, or at 

 the peak. The signal of a wish to communicate at night is given by- 

 rockets or Bengal lights. A ship signalizing with flags is represented in 

 pi. 26, Jig. 1 ; night signals in pi. 25, Jig. 5, a, b, c. 



Harbors, Naval Arsenals, and Light-Houses. 



A. Harbors. 



In order to furnish a safe berth for vessels when they are not at sea, for 

 the purpose of taking in or discharging their freight, a place is necessary 

 where they can find a good anchorage and a secure protection from storms. 

 Such places on the sea-coast are called harbors. A good harbor must have 

 a situation suitable for its objects, whether intended for men-of-war or for 

 merchant vessels. This is the first requisite. It must, in the next place, 

 be entirely protected from storms by the adjacent coast. Lastly, it must 

 possess a sound, tenacious bottom of clay or mud. A sand bottom also may 

 be used, but a rocky one is wholly impracticable. If a harbor gets stopped 

 up with sand, it must be dredged, for which purpose two different kinds of 

 machines are used. One is the common dredging machine (pi. SI, Jig. 6) 

 which is most serviceable on flat and gravelly bottoms. It consists of a 

 flat-bottomed boat, which is towed by a vessel to the place where it is to 

 be used. The tow-ropes run on a windlass, so that the position of the boat 

 can be changed without tacking the ship. The main part of the machine 

 is a large wheel, with a rope winding on its axis, by which the dredging- 

 box is raised. This is fastened to a long handle, which also rests on the 

 axis of the wheel. As the rope unwinds, the dredging-box sinks to the 

 ground, taking a part of the bottom as it is drawn away. It is then brought 

 to the surface of the water by the further revolution of the wheel, its con- 

 tents are discharged into a boat, and the same process is repeated. A far 

 more effective machine is the steam -dredger, of which pi. 30, fig. 8, ex- 

 hibits a view seen from above, and fig. 9 a longitudinal section. The 

 foundation of this also is a flat-bottomed boat, in which is the steam-boiler 

 A, with the safety-valve C, and the chimney D. The steam passes from 

 the boiler to the engine B, which by means of a fly-wheel b, and different 

 cog-wheels, turns the axis m. At the end of the axis, a mitre-wheel 

 moves the wheel F, on the pentagonal axis of which the bucket-chain FF 

 revolves, bringing the sand to the surface, where it is discharged into the 

 mud-scow G. By means of the chain t, which passes over the beam o, of 

 the windlass g, the bucket-chain can be raised or lowered by the pulley J9. 

 Artificial harbors like those of Dunkirk and Toulon (pi. 32, Jig. 2) require 

 extensive and costly works, as the whole system of dams which inclose the 

 harbor must be erected from the bottom of the sea. As scarcely any use 

 can be made of sails in harbor, vessels are towed to the spot where they 

 are to cast anchor (pi. 30, fig. 2). In the vicinity of a harbor is usually 

 found. 



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