Withy. — On the Stability of Ships. 659 



be the case I will undertake to shift the centre of gravity 

 without removing any cargo from the forward portion into the 

 .after one. I will simply take 20 tons of cargo out of the mid- 

 ship end of the forward part and stow it in the forecastle. 

 Common experience tells us that the result would be to trim 

 the vessel a little more by the head. At the same time, it 

 • would not add to the total weight of the forward portion of the 

 ship. But it would give the same weight a greater power of 

 leverage upon the after part. This increased power, which is 

 not due to added weight but to redistribution of what weight 

 was already there, is known as an increase of moment. It has 

 clearly moved the centre of gravity nearer to the bow, and 

 therefore away from the point which had been assumed to be 

 the dividing-line of equal weights. It can no longer be con- 

 tended, therefore, that the centre of gravity in a ship is the 

 dividing-point of equal weights. Her case is the same as that 

 of a steelyard, by means of which we can weigh large and 

 small packages by moving a small weight along a graduated 

 arm. It is the "moment" of the small weight acting at 

 different parts of a long lever which enables it to balance at 

 one time the small and at another the large package ; and 

 no one would contend that if the steelyard and its respective 

 loads were separated at the fulcrum, which is the common 

 centre of gravity of the whole, the two portions would be 

 equal in weight. 



I have dealt rather fully with this question of moment, 

 because a true appreciation of it is necessary at every point 

 in the consideration of stability. 



It will now be desirable to treat of a similar point to the 

 centre of gravity in connection with the supporting-power of 

 water. -It is known as the "centre of buoyancy." This 

 term is applied to the centre of gravity of the water displaced 

 by a ship. It may promote a thorough understanding of the 

 buoyancy question if I deal with the law of flotation by 

 means of several suppositions. Let us suppose that when a 

 vessel is floating in still water we could freeze the water all 

 aromid and under her,' and that it was then possible to lift 

 her completely out of her bed of ice ; the cavity from which 

 she had been lifted would be the space from which she had 

 expelled or " displaced " the water ; the cubic contents of this 

 cavity would represent the measure of what is called her 

 "displacement." To put it in another way: Let us suppose 

 the cavity to be gradually filled with water, measured into it 

 ton by ton, then the number of tons which were required to 

 completely fill it would be spoken of as the number of " tons 

 displacement " which the ship possessed at the draught of 

 water at which she was floating. This amount would corre- 

 spond in weight to the weight of the ship and her contents. 



