670 Transactions. — Miscellaneous. 



should be remembered that I have only dealt with stability in 

 still water, because it will readily appear that several of the 

 statements made are not applicable to a vessel's movements in 

 a seaway and in stress of weather, nor would they apply to 

 the case of a vessel anchored, or moored bow and stern 

 between buoys in a tideway. But even in still water the 

 pressure of a wind which careens a ship may not act in a 

 direction precisely parallel to the water-surface. In such a 

 case, the effect may be either to slightly lift or slightly depress 

 the ship, and thus cause an equivalent change of displacement. 

 If this ever happens, as it may do, then the solids of immer- 

 sion and emersion are to that extent not exactly equal to each 

 other. For practical purposes, however, this need not be 

 considered. 



There is a peculiarity about these solids, however, which is 

 well worth notice. Each of them has a centre of gravity of 

 its own, and it will very often happen that these are not 

 located at the same point, in a fore-and-aft direction, as that 

 of the main displacement when the ship is upright. "When 

 such is the case it is evident that the centre of buoyancy when 

 the ship is careened will not be in the same position as it was 

 when she was upright. The balance of the ship is thus dis- 

 turbed in the fore-and-aft direction, and the visible effect will 

 be that she will either rise at the bow and sink at the stern, 

 or else do exactly the reverse. It is evident, therefore, that a 

 vessel defectively designed in this particular (for it is a defect) 

 will, with every careening movement, combine a certain amount 

 of pitching and scending. If this happens in still water it is 

 likely to increase the liveliness of the vessel amongst waves 

 at sea. I should, speaking from my own feelings at least, 

 consider this quite an unnecessary aggravation. 



Another reservation was necessarily made once or twice re- 

 garding the exceptional actions of a body with a circular section. 

 You see when I place such a model in water that it has no 

 stability, and it may therefore be as well to explain the reason 

 of this. Owing to its being a homogeneous body, its centre of 

 gravity lies in its central axis. Again, owing to its circular 

 section, the act of careening does not alter the form of the 

 under-water portion. As a consequence, the solids of immer- 

 sion and emersion are exactly alike, not merely in bulk 

 (which is always the case), but in form also, and therefore in 

 moment. The result is that their addition and deduction 

 does not affect the position of the main centre of buoyancy, or 

 make it move out to leeward as usual. It therefore remains 

 vertically below the centre of gravity of the model, so that no 

 righting-lever is formed, and therefore no resistance is offered 

 to an inclining force, and no effort made to return to the 

 former position. 



