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rudiments of Mechanics,—and should know what is meant by the 
Centre of Buoyancy of a vessel. A few other terms are commonly 
employed in treating of sailing vessels, such as Centre of Lateral 
Resistance, Centre of Pressure, Metacentre ; these will present no 
difficulty when the first are clearly understood. 
As to the term Centre of Gravity, it is one used in common 
conversation, and usually correctly enough, if not with exact 
accuracy. It is currently defined thus: ‘‘The Centre of Gravity 
of a body is the point on which the body will balance in all 
positions”; or we may say that it is the point at which the whole 
mass of the body may be supposed to be collected, or the whole 
weight of the body supposed to act. In speaking of the Centre of 
Gravity ofa boat, I must beunderstood to mean the Centre of Gravity 
of the boat including all that she carries, the masts, sails, crew, 
and everything on board. It is obvious that in a light boat the 
position of the Centre of Gravity will be considerably affected by 
the movements of those on board, that, for instance, if they all 
stand up, it will be materially raised; whereas in a heavily 
ballasted boat it will be but slightly deranged. 
The term Couple is less generally used, and may require a 
little explanation. A Couple consists of two equal forces acting 
in opposite directions, but not through one point. (See Fig. 1.) 
In working a press, the forces applied at the two ends of the lever 
or handle constitute a couple. The effect of a couple is simply to 
turn the body on which it acts round, not to move it forwards in 
space. And conversely in Mechanics every motion of rotation, ° 
every motion by which a body revolves without advancing or 
receding, is conceived of as the effect of a couple. 
The power of a couple depends on two things : the intensity 
of the forces, and the distance between their lines of action. This 
will be evident at once to any one who has ever tried to turn a 
press. The distance between the lines of action must be measured 
from one line to the other along a straight line perpendicular to 
both, as AB in Figure 1. It is called in Mechanics the Arm of 
the Couple; and in all questions of the stability of boats, the arm of 
the righting couple is an element of the first importance, 
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