the centre, the head of it, which is farther away
from that centre, on being depressed, is made to describe a circular
movement, and thus by pressure brings to an equilibrium the weight of a
very great load by means of a few hands. Again, if the tongue of an iron
lever is placed under a weight, and its head is not pushed down, but, on
the contrary, is heaved up, the tongue, supported on the surface of the
ground, will treat that as the weight, and the edge of the weight itself
as the fulcrum. Thus, not so easily as by pushing down, but by motion in
the opposite direction, the weight of the load will nevertheless be
raised. If, therefore, the tongue of a lever lying on a fulcrum goes too
far under the weight, and its head exerts its pressure too near the
centre, it will not be able to elevate the weight, nor can it do so
unless, as described above, the length of the lever is brought to
equilibrium by the depression of its head.

4. This may be seen from the balances that we call steelyards. When the
handle is set as a centre close to the end from which the scale hangs,
and the counterpoise is moved along towards the other arm of the beam,
shifting from point to point as it goes farther or even reaches the
extremity, a small and inferior weight becomes equal to a very heavy
object that is being weighed, on account of the equilibrium that is due
to the levelling of the beam. Thus, as it withdraws from the centre, a
small and comparatively light counterpoise, slowly turning the scale,
makes a greater amount of weight rise gently upwards from below.

5. So, too, the pilot of the biggest merchantman, grasping the steering
oar by its handle, which the Greeks call [Greek: oiax], and with one
hand bringing it to the turning point, according to the rules of his
art, by pressure about a centre, can turn the ship, although she may be
laden with a very large or even enormous burden of merchandise and
provisions. And when her sails are set only halfway up the mast, a ship
cannot run quickly; but when the yard is hoisted to the top, she makes
much quicker progress, because then the sails get the wind, not when
they are too close to the heel of the mast, which represents the
centre, but when they have moved farther away from it to the top.

6. As a lever thrust under a weight is harder to manage, and does not
put forth its strength, if the pressure is exerted at the centre, but
easily raises the weight when the extreme end of it is p