ushed down, so
sails that are only halfway up have less effect, but when they get
farther away from the centre, and are hoisted to the very top of the
mast, the pressure at the top forces the ship to make greater progress,
though the wind is no stronger but just the same. Again, take the case
of oars, which are fastened to the tholes by loops,--when they are
pushed forward and drawn back by the hand, if the ends of the blades are
at some distance from the centre, the oars foam with the waves of the
sea and drive the ship forward in a straight line with a mighty impulse,
while her prow cuts through the rare water.

7. And when the heaviest burdens are carried on poles by four or six
porters at a time, they find the centres of balance at the very middle
of the poles, so that, by distributing the dead weight of the burden
according to a definitely proportioned division, each labourer may have
an equal share to carry on his neck. For the poles, from which the
straps for the burden of the four porters hang, are marked off at their
centres by nails, to prevent the straps from slipping to one side. If
they shift beyond the mark at the centre, they weigh heavily upon the
place to which they have come nearer, like the weight of a steelyard
when it moves from the point of equilibrium towards the end of the
weighing apparatus.

8. In the same way, oxen have an equal draught when their yoke is
adjusted at its middle by the yokestrap to the pole. But when their
strength is not the same, and the stronger outdoes the other, the strap
is shifted so as to make one side of the yoke longer, which helps the
weaker ox. Thus, in the case of both poles and yokes, when the straps
are not fastened at the middle, but at one side, the farther the strap
moves from the middle, the shorter it makes one side, and the longer the
other. So, if both ends are carried round in circles, using as a centre
the point to which the strap has been brought, the longer end will
describe a larger, and the shorter end a smaller circle.

9. Just as smaller wheels move harder and with greater difficulty than
larger ones, so, in the case of the poles and yokes, the parts where the
interval from centre to end is less, bear down hard upon the neck, but
where the distance from the same centre is greater, they ease the burden
both for draught and carriage. As in all these cases motion is obtained
by means of right lines at the centre and by circles, so also farm
waggons