206 Prof. F. Elgar. Tlie Variation of 



stability has its own peculiar objections and dangers. It causes heavy 

 and uneasy rolling which may not only be uncomfortable and trying 

 to passengers and crew, but at times sufficient to strain and even 

 fracture portions of the structure, and displace or damage some of 

 the fittings. The masts and rigging generally suffer most on account 

 of their distance from the axis of rotation, and the change of motion 

 in them at the end of each roll, being greater than that of any other 

 part of the vessel ; and also because their attachment to the hull is 

 less direct and secure. The straining effect of too great stability 

 often shows itself early, and in a marked manner, upon the masts and 

 rigging of a vessel. Seamen frequently say of a ship that is laden with 

 heavy cargo placed low down in the hold, that unless some of the 

 weights are raised, i.e., unless the stability is reduced, she will roll 

 her masts out. The violent and deep rolling caused by excessive 

 stability also tends to move over towards one side of the ship such 

 cargoes as grain or coals which are free, to some extent, to shift as a 

 whole ; or snch portions of other classes of cargo as admit of being 

 displaced. Speaking generally, it may be said that, whereas large 

 stability helps to prevent inclination to a great angle in the event 

 of cargo shifting, 'the very possession of such large stability often 

 increases the chances of shifting. 



The stability of a ship at a given draught of water, and with a 

 specific description of loading can readily be calculated, and it has 

 become usual to make such calculations at the Admiralty and in a 

 few of our leading mercantile shipyards. The practice is to construct 

 a curve of metacentres, which shows how the height of the meta- 

 centre varies with draught of water, and is affected by such changes 

 in the external bulk and form of the immersed portion of the hull 

 as may be caused by increasing or diminishing the draught of water. 

 A curve of stability is also constructed for one or more fixed draughts 

 of water, and for certain intended, or estimated, weights and distri- 

 butions of load. These curves show what the righting moments for 

 snch draughts of water and descriptions of loading amount to at 

 successive angles of inclination from the upright up to 90, or to the 

 angle at which the stability vanishes. 



The curve of metacentres is obtained by calculating the height of 

 the metacentre at several fixed draughts of water, setting up those 

 heights as ordinates of a curve whose abscissae are proportional to 

 the corresponding draughts of water, and drawing a fair curve 

 through the points thus obtained. The term " metacentre " wad 

 originated by Bouguer, who published tho first investigations into the 

 subject in his " Traite du Navire," which appeared in 1740. The 

 metacentre of a floating body is the point of intersection of a vertical 

 line through the centre of gravity of the volume of displacement 

 when the body is inclined through an indefinitely small angle from a 



