38 REPORT— 1869. 



plates would tend to make any marked alteration in the invariability of 

 direction of the bisectors, the theoretical establishment of this fact is of 

 great importance. Its meaning is that the effect of bilge-keels is to increase 

 the time, and, in a greater degree still, to diminish the amplitude of the 

 oscillation, and that the use of bilge-keels is the direct mode of effecting 

 this object. 



The problem of safe rolling is not quite the same with that of easy 

 rolling. A roll towards the wave- crest is well known as one of the most 

 dangerous things that can happen to a ship in a high-crested sea-way, for 

 the whole crest of the wave may then break inboard. Even when the ship 

 follows the oscillations of the vertical lines, the wave-particles come flat on 

 the ship's bulwarks and side. If she floats quite vertically she is still in the 

 position of a cliff resisting a wave of the same period, whose height is the 

 difterence of heights of the surface-wave and of the m^can effective wave 

 acting upon her. 



As regards the impact of a wave, the most violent blow that a wave can 

 give is against a surface parallel to the inflexional tangent and to the wave- 

 crest, and at a level with the line of inflexion. The motion of the particles 

 is then normal to the wave-surface. This remark, of course, does not apply 

 to shore-waves. 



Throughout the discussion of the ship's oscillation among waves, it has 

 been tacitly assumed that the wave-period itself might be regarded as con- 

 stant. This is very far from either representing the facts or the practical 

 problem of the shipbuilder. The wave which a vessel has to encounter may 

 be anything, from the 11-seconds wave, 600 feet long, to a mere ripple. 

 Practically, a vessel will not roll to waves whose length is much less than 

 her breadth, nor will she pitch much among short waves. But, dismissing 

 these from consideration, it may still be impossible to avoid some contin- 

 gency in which a ship's period of free rolling may be equal to the wave- 

 period. Obviously, the remedy in this case is for her commander not to 

 keep her hroadskle-on. As a rule, no commander ever would do so in a 

 dangerous sea-way ; and even where comfort only is concerned, it is usually 

 open to him either to shorten the effective (that is to say, the apparent 

 wave-period) by putting her head a little to the swell, or to lengthen the 

 apparent wave-period by putting her head a little ofi'. He must do one of 

 these things if he meets with actual and exact synchronism in anything like 

 heavy weather. 



As a practical matter. Professor Rankine remarks : " It would appear 

 that a very close approximation to the form and proportions which are 

 most favourable to steadiness has, in some cases, been realized by practical 

 trials alone, and that independently of the steadying action of sails ; for 

 there are vessels which, when under steam alone, in any moderate swell 

 keep their decks very nearly parallel to the horizon. It is of great im- 

 portance that the lines and dimensions, and distribution of the weights of 

 ships, which have been found by experience to possess tliis excellent quality, 

 should be carefully recorded for the information of naval architects. 



" On the other hand, there are vessels (especially screw- steamers) whose 

 ordinary extent of rolling each way is from three to four times the slope of 

 the waves." 



On the subject of Waves, we refer to the following papers and treatises : — 



Weber, ' Wellenlehre.' 



Aiiy, " On Tides and Waves," Encycl. Metropolitana (reprinted in a 

 separate form). 



