STEAM NAVIGATION. 



piston area equal to that of a piston 55 inches in diameter. In the ' 'Arrogant" 

 the length of stroke is 3 feet, and in the "Encounter" it is 2 feet 3 inches. 

 The nominal power of both engines is 360 horses ; and the diameter of the 

 "Arrogant's" screw is 15 feet 6 inches, that of the "Encounter" being 12 feet. 

 The pitch of both is 15 feet, and the length 2 feet 6 inches. The * 'Arrogant" 

 Is a vessel of 1872 tons burden, and the "Encounter" of 953 tons. The 

 whole machinery, including the boilers, is placed below the water line, so 

 as to be protected from shot.* 



88. The forms of screw-propelling engines, whether they act on 

 the screw-shaft by intermediate gearing or directly, are infinitely 

 various. Drawings of not less than 15 different forms of geared- 

 engines, and the like number of direct acting engines, are given 

 in two large plates prefixed to Mr. Bourne's work on the screw- 

 propeller, to which we must refer those who require information 

 of this detailed description. In the vessels of the Royal Marine 

 generally the cylinders are placed upon the sides, so that, by 

 diminishing the total height of the machinery above the floor 

 on which it rests, it may be kept below the water-line. In 

 commercial vessels a form of engines is frequently employed 

 resembling the land beam-engines, with the cylinder at one 

 end of the beam, and the connecting-rod at the other. In such 

 cases the connecting-rod extends downwards from the end of 

 the beam to the crank. In either case the cylinder is inverted, 

 and the connecting-rod proceeds from the end of the piston-rod 

 to turn the crank, the end of the piston-rod being of course 

 steadied by suitable guides. According to Mr. Bourne, the con- 

 struction of the engines described above in the case of the 

 * 'Arrogant" and " Encounter " is, on the whole, the best for screw- 

 vessels, but he thinks it might be preferable to put the trunk 

 into the air-pump instead of the cylinder. He considers also that 

 the condenser might be dispensed with, and the condensations 

 performed in the air-pump. In that case the flow of water to 

 and from the air-pump might be governed by a slide-valve, 

 similar to that which is employed to regulate the admission and 

 escape of steam to and from the cylinder. It seems probable that 

 slide-valves may be brought into general use for pumps of every 

 sort, but in the case of ordinary ones for raising water these valves 

 need not be like the common slide-valves, which in fact are not 

 well adapted to give sufficient area for such purposes, but may 

 consist of a short wide cylinder with gridiron orifices revolving 

 slowly at the top and bottom of the air-pump. 



89. The general arrangement of the machinery and fuel in screw- 

 propelled vessels of the Royal Navy is illustrated by the transverse 

 section of H.M.'s screw steam-packet "Plumper," shown in fig. 28. 



* Figs. 26 and 27 are copied, with the permission of the publisher and 

 the author, from Brande's "Dictionary of Science and Art," to which the 

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