152 



NATURE 



[June 15, 1882 



ments by means of bulkheads. A longitudinal bulkhead 

 is fitted at the middle line ; the boilers are contained in 

 four separate water-tight compartments ; the engines in 

 two; and the coal bunkers are also water-tight. 



The engines and boilers are manufactured by Messrs. 

 Humphreys and Co. There are two pairs of engines 

 working twin screws. They are of the horizontal com- 

 pound type, the cylinders of each pair being 38 inches 

 and 64 inches in diameter, and the stroke 39 inches. 

 They are intended to indicate an aggregate horse power 

 of 5500. These engines are remarkable for their lightness 

 and the comparatively small space they occupy. Most of 

 their novel features have been adopted for the purpose of 

 economising weight and space. They are almost entirely 

 of wrought iron, Whitworth steel, and gun-metal ; very 

 little cast iron being used in their construction. The 

 screws are three-bladed, and are 14 feet in diameter, with 

 15 feet to 17 feet pitch. The shafts are left bare where 

 they come outside the hull of the ship, and are not sur- 

 rounded by tubes, as is usual in ships of the Navy. 

 These tubes have been dispensed with for the purpose of 

 diminishing the resistance. The boilers are of the loco- 

 motive type ; and these also were adopted in preference to 

 the ordinary marine boiler, for the purpose of saving 

 weight and space. They are 5 feet 3 inches in diameter, 

 and 14 feet 4 inches in length, and work with a steam 

 pressure of 120 tons per square inch. The shells are of 

 steel, the fire-boxes of iron, and tubes of brass ; and they 

 are similar to ordinary locomotive boilers, except that the 

 tubes are shorter and the fire wells less deep. The stoke- 

 holds are closed in, and forced draught is worked with, as 

 in the fast torpedo boats. This is supplied by four fans, 

 two of which are 4 feet and the other two 3 feet 6 inches 

 in diameter. The fan engines have 9-inch cylinders, and 

 4| inches stroke, and run at the rate of 900 to 1000 revo- 

 lutions per minute when working at full speed. 



The trials of the machinery have, so far, not been suc- 

 cessful, chiefly on account of difficulties with the boilers. 

 During the first series of trials, on March 2, 4, and 6 last, 

 nothing could be done on account of priming. The 

 greatest speed realised was 10 to 12 knots, when the 

 boilers primed so badly that a stop had to be come to. 

 On March 31 there was another trial, the last down to 

 the present time, when the difficulties of priming were 

 mainly got over. The indicated horse-power on that 

 occasion was about 5000, and the speed a little over 17 

 knots. The air-pressure in the stoke-holds, which gave 

 the forced draught, was equivalent to 5 inches of water. 

 On this occasion the boiler-tubes leaked very badly, so 

 that the full power could not be realised. The priming 

 was due to oil from the engines getting into the boilers, 

 and this now appears to be remedied. In the torpedo 

 boats that are fitted with locomotive boilers, the same 

 difficulty arises, and oil is not used at all in the cylinders, 

 or only very sparingly. The leaking of the tubes is a 

 more serious difficulty to get over, although in the Poly- 

 phemus the arrangements appear to admit of improve- 

 ment. For instance, solid iron stays were fitted in the 

 midst of the brass tubes ; and it must be obvious that the 

 unequal rate of expansion of the stays and tubes when 

 heated to a high temperature must have considerably 

 strained the tube plates. These stays are now being 

 removed, and new tubes are being fitted throughout, the 

 ends of which are to be screwed into the tube plates. 



This difficulty of leaky tubes is not peculiar to the 

 Polyphemus. Messrs. Thornycroft are in the same posi- 

 tion with a large number of torpedo boats they have 

 completed for the British Government, and which arc 

 undergoing a similar ordeal of testing by the Admiralty 

 engineers. These boilers cannot be got to stand satis- 

 factorily, and a number of experiments have just been 

 carried out at Portsmouth upon tubes fitted in various 

 ways in a torpedo boat boiler, which, it is hoped, will 

 show how the present defects can be remedied. Loco- 



motive boilers are not adapted for working continuously 

 at a high rate, and for steaming at full speed over long 

 distances. The strain put upon the boiler, and the work 

 attempted to be got out of it, is too great under these 

 circumstances. What is being done in the Polyphemus 

 and in the torpedo boats, is to get the advantage of the 

 lightness of this type of boilers ; and only to press them 

 up to their full power for comparatively short times when 

 required in an emergency. 



The armour plating is of steel ; and here again we find 

 an attempt to combine great defensive power with ex- 

 treme lightness. It extends over the whole of the above 

 water-portion of the hull, and for a short distance below 

 the water-line. There are first two half-inch thicknesses 

 of Landore-Siemens steel, upon which are placed plates 

 of Whitworth fluid-compressed steel, one inch thick. 

 Outside of this is another layer of hard Whitworth steel, 

 one inch thick, which is tested to a strain of sixty-eight 

 tons per square inch. This outer layer is fitted in small 

 plates or scales ten inches square, secured with coned 

 steel screw bolts, one at the centre of each of the plates, 

 and one at each of the corners. Along the middle of the 

 turtle-back deck these scales are omitted, and the armour 

 is there only 2 inches thick. The bases of the trunks 

 from the hatchways to the flying deck are protected bv a 

 glacis of 6-inch steel armour to a height of 3 feet 6 inches 

 above the deck ; and the front of the foremost trunk is 

 plated to a height of about 5 feet above the flying deck, 

 with 8 inches of steel-faced armour, which gives protec- 

 tion to the pilot tower. 



The armament, as has been stated, merely consists of six 

 Nordenfelt machine-guns, which are each mounted in a re- 

 volving turret that projects from the side of the flying deck. 

 The fighting weapons she possesses are the ram and torpe- 

 does. The former is very long and strongly constructed. 

 It is, however, interfered with by a tube for ejecting torpe- 

 does right ahead, which is fixed in the centre of the ram. 

 This seems a doubtful expedient to adopt, and to en- 

 danger to some extent both the ram and the torpedo-tube. 

 The bow has been made so as to protect this tube as 

 much as possible. The torpedo armament consists of 

 the tube referred to for ejecting torpedoes right ahead, 

 and of two tubes on each side, also placed under water, 

 in a compartment at the fore side of the boiler-rooms. In 

 this torpedo compartment one tube on each side is fixed 

 right abeam, and the other in the direction of about 20 

 degrees at the fore side of the beam. There is only means 

 of firing one torpedo end on, the other four tubes being 

 on the broadside. It is questionable as to the merits of 

 this arrangement, considering that the end-on position 

 will be the one for attacking from in the Polyphemus, for 

 the purpose of using the ram, and also to enable the 

 greatest possible resistance to be got out of her thin 

 armour. Independent air-compressing machinery for 

 the torpedoes is carried in each of the torpedo-chambers, 

 in which also a number of torpedoes will be carried 

 ready for use. 



The height of the hull proper above the water line is, 

 as we have said, 4 feet 6 inches. It is kept low to reduce 

 the chances of penetration ; but to furnish more buoyancy 

 than this small freeboard gives, a strange device has been 

 adopted. At the keel of the ship a deep rectangular recess 

 is made in which about 300 tons of iron ballast is carried. 

 This ballast is so fixed that it can be let go at pleasure, 

 and the ship lightened accordingly. The draught and 

 trim may thus be regulated to some extent should the 

 ship be injured in action. If the whole of t'.ie ballast is 

 let go it will lighten her about 14 inches. 



As manoeuvring power is of great importance to such a 

 vessel as the Polyphemus, an attempt is made to increase 

 it by means of bow rudders. Two of the rudders, of the 

 balanced form, are placed forward, one on each side of 

 the bow torpedo tube. They can be drawn up into aper- 

 tures inside the hull when not required for use ; and when 



