SCIENCE. 



[Vol. XIX. No. 482 



The two most obvious modes of preventing heavy rolling 

 are, therefore, (1) to make the period of rolling of a ship as 

 long as possible, so as to reduce the chances of meeting waves 

 whose period will synchronize with it, and (2) to increase 

 the resistance to rolling. The period of a ship varies di- 

 rectly as her radius of gyration, and inversely as the square 

 root of her metacentric height. Hence the period may be 

 increased by increasing the moment of inertia of the ship, or 

 by decreasing the metacentric height. In armored war-ves- 

 sels the moment of inertia is large, on account of the heavy 

 weights of armor on the sides, and the heavy guns that are 

 either placed at the side or high up above the centre of 

 gravity. Ordinary steamers have no such weights concen- 

 trated at gi-eat distances from the centre of gravity, and their 

 moments of inertia are determined by the distribution of 

 material in the hull that is fixed by structural conditions and 

 by the stowage required for their voyages. Metacentric 

 height cannot be reduced below a certain amount, which is 

 necessary to prevent too easy inclination of the ship, or 

 crankness, in still water. On the whole, we may regard the 

 longest periods that the largest ships are likely to have with 

 advantage to be about those named above, i.e., fifteen to 

 eighteen seconds. 



Length of period cannot give immunity against occasional 

 heavy rolling; but increase of resistance reduces the angles 

 of roll at all times, and especially when the angular velocity 

 is greatest and the rolling is worst. Such resistance is fur- 

 nished by the frictional resistance of the bottom of a ship 

 and by the direct resistance of projecting parts of the bottom, 

 such as the keel and the large flat surfaces below at the stem 

 and stern. This resistance can be largely increased by means 

 of bilge keels. The value of bilge keels is recognized in the 

 Royal Navy, and the ships of the navy have been fitted with 

 them for many years with highly beneficial results. The 

 advantage of bilge keels was proved beyond all doubt many 

 years ago by careful experiments made in this country and 

 in France; and the late Mr. Wm. Froude showed, by the 

 trials he made of H.M.S. "Greyhound" twenty years ago, 

 that bilge keels of excessive size — 3 feet six inches deep, 

 and 100 feet in length, on a vessel 172 feet long — had only 

 an insignificant effect upon speed throughout great differ- 

 ences of trim. 



It is strange that the mercantile marine should not yet 

 have adopted bilge keels, and obtained the undoubted ad- 

 vantage they give in steadiness. The number of ships that 

 have them is comparatively few. There is an almost uni- 

 versal opinion and prejudice against their use, and the 

 largest and finest passenger steamers have no bilge keels. 

 This is in spite of the fact that, in cases where bilge keels 

 have been fitted to try to check heavy rolling — and they 

 have been of suitable size and properly placed — it has been 

 found that the angles of rolling have been reduced by nearly 

 one-half. There is a prevalent belief — which has no foun- 

 dation in fact — that bilge keels are very detrimental to 

 speed. We have said that Mr. Froude's experiments showed 

 the contrary, even on trials made in still water; but it ap- 

 pears certain that at sea any trifling loss of speed which 

 still-water trials might show would be more than compen- 

 sated for by gain in speed when the vessel is prevented from 

 rolling through large angles from side to side, and undergo- 

 ing great changes of underwater form at every roll. Experi- 

 ence vfith ships that have had bilge keels added after running 

 for some time without them shows that there has been no 

 appreciable difference of speed or increase of coal consump- 

 tion on their voyages. 



Another, and a more heroic, method of stopping or reduc- 

 ing rolling would be to counteract the inclining moment of 

 the ship caused by the ever-changing inclination of the 

 waves by an equal and opposite moment, which would vary 

 as the inclining moment varies. This has been attempted at 

 different times and in various ways. It is essential to any 

 degree of success, however, that the opposing moment brought 

 into operation should be completely under control, so as 

 always to act in the manner and to the extent required. The 

 attempts to obtain a steady platform by freely suspending it, 

 and making it independent of the rolling of the ship, have 

 failed — apart from the practical difficulties of carrying out 

 such an arrangement on a large scale — because the point of 

 suspension oscillates when the ship rolls, and the platform 

 acquires a rolling motion of its own. Weights, made of 

 heavy solid material, which move from one side to the other 

 of a ship subject to the action of gravity and rotation, fail 

 because they cannot be made to act continuously in the man- 

 ner required. 



A degree of success has been achieved by admitting water 

 into a suitably prepared chamber and leaving it free to move 

 from side to side as the ship rolls. This has been done in 

 several ships of the navy, the case of the " Inflexible " being 

 that which was the most carefully experimented upon. The 

 movement of this internal water follows the inclination of 

 the ship, but it lags behind, and thus tends to reduce the in- 

 clination. Its effect can be regulated by the quantity of 

 water admitted into the chamber and by its depth. The 

 "Inflexible" committee state in their report that compara- 

 tively small changes in depth increase or diminish largely 

 the extinctive power of the water. For various reasons — 

 one of which is that while such a chamber is very effective 

 in a moderate sea it fails in a rough sea when the rolling of 

 the ship is greatest — and perhaps partly on account of the 

 destructive and disturbing effect of 100 tons or more of vrater 

 rushing from side to side of a ship over sixty feet wide — 

 these water-chambers appear to have gone out of use in the 

 navy, and they have been given up in the "City of New 

 York" and " City of Paris," which vessels were said to be 

 fltted with them when first built and placed upon the At- 

 lantic. 



Mr. Thornycroft has devised a means of checking rolling 

 by moving a weight, under strict control, from side to side 

 of a vessel so as to continuously balance, or subtract from, 

 the heeling moment of the wave-slope. It consists of a large 

 mass of iron in the form of a quadrant of a circle, which is 

 placed horizontally, with the centre on the middle line of the 

 vessel, and there connected with a vertical shaft. The shaft 

 is turned by an hydraulic engine, which is very ingeniously 

 controlled by an automatic arrangement. The heavy iron 

 quadrant is swept round from side to side, revolving about 

 its centre, to the extent that is required to counteract the 

 heeling moment. In a paper read on the 6th instant be- 

 fore the Institution of Naval Architects, Mr. Thornycroft 

 said : — ■ 



"The manner in which the controlling gear works will 

 be better understood if we imagine a vessel remaining up- 

 right among waves, while near the centre of gravity of the 

 ship we place a short-period pendulum suspended so as to 

 move with little friction; this will follow the change in the 

 apparent direction of gravity without appreciable loss of 

 time, so that any change in the wave- angle and apparent 

 direction of gravity cannot take place without due warning, 

 which will indicate the time and amount of the disturbance. 

 It is therefore only necessary to make the motion of the 



