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the mast, which is as it were the centre, but at a distance
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, raiséd 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.
Also, when 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 pro-
portion of the burden. The middle parts of the levers
for four persons over which the tackle passes, are pro¬
vided 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 also oxen have an equal draft 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 apportioning to each his due labour is by
shifting the suspending piece so that one side of the yoke
shall be longer than the other, and thus relieve the weak-
er animal. It is the same in the porters' levers as in
yokes, when the suspending tackle is not in the centre,