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
intersection. Then it is carried across obliquely to the succeeding
intersections of longitudinal lines and circles, and as it advances,
passing each of the points in due order and winding round, it is
fastened at each intersection; and so, withdrawing from the first to the
eighth point, it reaches and is fastened to the line to which its first
part was fastened. Thus, it makes as much progress in its longitudinal
advance to the eighth point as in its oblique advance over eight
points. In the same manner, withes for the eight divisions of the
diameter, fastened obliquely at the intersections on the entire
longitudinal and peripheral surface, make spiral channels which
naturally look just like those of a snail shell.
3. Other withes are fastened on the line of the first, and on these
still others, all smeared with liquid pitch, and built up until the
total diameter is equal to one eighth of the length. These are covered
and surrounded with boards, fastened on to protect the spiral. Then
these boards are soaked with pitch, and bound together with strips of
iron, so that they may not be separated by the pressure of the water.
The ends of the shaft are covered with iron. To the right and left of
the screw are beams, with crosspieces fastening them together at both
ends. In these crosspieces are holes sheathed with iron, and into them
pivots are introduced, and thus the screw is turned by the treading of
men.
4. It is to be set up at an inclination corresponding to that which is
produced in drawing the Pythagorean right-angled triangle: that is, let
its length be divided into five parts; let three of them denote the
height of the head of the screw; thus the distance from the base of the
perpendicular to the nozzle of the screw at the bottom will be equal to
four of those parts. A figure showing how this ought to be, has been
drawn at the end of the book, right on the back.
I have now described as clearly as I could, to make them better known,
the principles on which wooden engines for raising water are
constructed, and how they get their motion so that they may be of
unlimited usefulness through their revolutions.