"Withy. — On the Stability of Ships. 671 



We can, however, give stability to a model of this section 

 by loweriug its centre of gravity. If a small weigbt is attached 

 to the lower pai't of the model it will do this. If it is then 

 careened the centre of gravity will move out from the up- 

 right centre-line, up which the centre of buoyancy still acts, 

 and will thus form a righting-lever. This lever would con- 

 tinue to increase in length up to an angle of 90°, and the 

 stability would increase at the same rate. The model, under 

 botli conditions, acts in the same way if placed on a plane 

 surface. The reason is that the point of support in both 

 cases acts in the vertical line falling from the centre of rota- 

 tion. Fig. 4 shows the direction of the forces for the weighted 

 model both in and out of water. 



Another way of giving stability to a model of circular 

 section without removing its centre of gravity from the 

 central axis is as follows : I will take off the weight used in 

 the last experiment, and attach a bolster or fender to each 

 side opposite to the centre-line, as shown in fig. 5, Plate LI. 

 It is obvious that the bolsters do not change the position of the 

 centre of gravity, but as soon as either of them touches the 

 water it affords stability. The reason, following the lines of 

 former explanations, is that the solid of immersion, while still 

 of the same volume as that of emersion, is, owing to one 

 bolster being immersed, of a different form, and therefore 

 possessed of a greater moment. It therefore draws the centre 

 of buoyancy to leeward, forms a righting-lever, and endows 

 the model with a measure of statical stability. A little 

 further observation will show that the centre of buoyancy has 

 risen vertically at the same time. This shows that the model 

 has also acquired dynamical stability. 



A device analogous to this addition of bolsters will occur to 

 some present as being used sometimes to give stability to a sail- 

 ing-vessel when it may be necessary to shift her after discharging 

 cargo. A square log of timber, such as a spare spar which 

 she may carry for the purpose of making a topmast or lower 

 yard from, is secured by two ropes and lowered over each side 

 into the water. After this, if the vessel takes a list she lifts the 

 windward balk out of the water, and leaves the other floating 

 free. The weight of the balk thus lifted influences the ship's 

 centre of gravity slightly downwards and to windward, and 

 therefore tends to right her. 



The effectiveness of this plan can be increased if neces- 

 sary by passing a rope tightly under the ship's bottom and 

 securing it to both balks. If she then takes a list she hauls 

 one balk down under water at the same time that she lifts 

 the other out. One effect of this modification is to draw the 

 centre of gravity rather more down, but not at all out to 

 windward. The other, and the principal effect, is to increase 



