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very heavy load. Thus also the steersman of a mer-
chant ship, holding the tiller which the Greeks call
olag with only one hand, by the situation of the centre
moves it in a moment as the nature of the case requires,
and turns the ship though ever so deeply laden. The
sails also, if only half mast high, will cause the vessel to
sail slower than when the yards are hoisted up to the
top of the mast, because not then being near the foot of
the mast, which is as it were the centre, but at a dis
tance therefrom, they are acted on by the wind with
greater force. For as, if the fulcrum be placed under
the middle of a lever, it is but with difficulty that the
weight is moved, and that only when the power is applied
at the extremity of the lever, so when the sails are no
higher than the middle of the mast, they have less effect
on the motion of the vessel: when, however, raised to
the top of the mast, the impulse they receive from an
equal wind higher up, causes a quicker motion in the
ship. For the same reason the oars, which are made
fast with rope to the thowls, when plunged into the water
and drawn back by the hand, impel the vessel with great
force, and cause the prow thereof to cleave the waves,
if their blades are at a considerable distance from the
centre, which is the thowl. When also loads of great
weight are carried by porters in gangs of four or six, the
levers are so adjusted in the middle that each porter may
be loaded with a proper proportion of the burden. The
middle parts of the levers for four persons over which
the tackle passes, are provided with pins to prevent it
sliding out of its place, for if it shift from the centre.
the weight will press more on the shoulders of him to
whom it is nearest, just as in the steelyard the weight is
shifted towards the end of the beam. Thus too, oxen
have an equal draught when the piece which suspends the
pole hangs exactly from the middle of the yoke. But
when oxen are not equally strong, the method of appor-
tioning to each his due labour is by shifting the sus
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