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SAILING BOATs. 
Sailing boats have to encounter a different class of risk from 
rowing boats, and must be constructed on altogether different 
principles. A rowing boat should always be kept on an even keel; 
a sailing boat is not usually doing her best till she lies over about 
thirty degrees. She is intended to sail at about this angle, and to 
withstand the capsizing power of the wind. 
In a fresh breeze the wind moves at the rate of about fifteen 
miles an hour, and exerts a pressure of about one pound on every 
square foot of surface directly exposed to it. This pressure 
becomes very much greater in a gale, increasing with the square 
of the velocity of the wind. It is said to have reached on some 
occasions to one hundred pounds per square foot; but the 
observations are a little doubtful. One pound, as I said, to the 
foot is about the pressure in a good breeze. The Windermere 
yachts spread about 1000 square feet of canvas, more or less; so 
that if these sails were held firmly in a plane perpendicular to the 
wind, there would be a pressure upon them of nearly half a ton. 
Practically, the motion of the boat and the inclination of the sails 
diminish the pressure very considerably. Of what remains, a part 
goes to drive the boat, and a part,—the greater part when the boat 
is on the wind,—to heel her over. The heeling of the boat, like 
every motion of rotation, is caused by the action of a couple, the 
one force being, as I have said, the pressure of the wind, the other 
the lateral resistance of the water. Just as in every body there is 
a point at which the whole weight of the body may be supposed 
to act, so in every surface there is a point at which the whole of 
any uniform pressure on that surface may be supposed to be 
applied. In the case of the sails of a boat this point is called the 
Centre of Pressure; in the case of the surface of the hull exposed 
to the lateral pressure of the water, it is called the Centre of 
Lateral Resistance. The couple which tends to heel the boat over 
consists then of the force of the wind acting at the Centre of Pressure 
of the sails, and the resistance of the water acting at the Centre of 
Lateral Resistance (see Fig. 7). The calculation of these centres 
