WATER WHEELS AND WATER MILLS

1. Wheels on the principles that have been described above are also
constructed in rivers. Round their faces floatboards are fixed, which,
on being struck by the current of the river, make the wheel turn as they
move, and thus, by raising the water in the boxes and bringing it to the
top, they accomplish the necessary work through being turned by the mere
impulse of the river, without any treading on the part of workmen.

2. Water mills are turned on the same principle. Everything is the same
in them, except that a drum with teeth is fixed into one end of the
axle. It is set vertically on its edge, and turns in the same plane with
the wheel. Next to this larger drum there is a smaller one, also with
teeth, but set horizontally, and this is attached (to the millstone).
Thus the teeth of the drum which is fixed to the axle make the teeth of
the horizontal drum move, and cause the mill to turn. A hopper, hanging
over this contrivance, supplies the mill with corn, and meal is produced
by the same revolution.

CHAPTER VI

THE WATER SCREW

1. There is also the method of the screw, which raises a great quantity
of water, but does not carry it as high as does the wheel. The method of
constructing it is as follows. A beam is selected, the thickness of
which in digits is equivalent to its length in feet. This is made
perfectly round. The ends are to be divided off on their circumference
with the compass into eight parts, by quadrants and octants, and let the
lines be so placed that, if the beam is laid in a horizontal position,
the lines on the two ends may perfectly correspond with each other, and
intervals of the size of one eighth part of the circumference of the
beam may be laid off on the length of it. Then, placing the beam in a
horizontal position, let perfectly straight lines be drawn from one end
to the other. So the intervals will be equal in the directions both of
the periphery and of the length. Where the lines are drawn along the
length, the cutting circles will make intersections, and definite points
at the intersections.

[Illustration: CONSTRUCTION OF THE WATER SCREW]

[Illustration: THE WATER SCREW

(From the edition of Vitruvius by Fra Giocondo, Venice, 1511)]

2. When these lines have been correctly drawn, a slender withe of
willow, or a straight piece cut from the agnus castus tree, is taken,
smeared with liquid pitch, and fastened at the first point of
int