ersection. 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.

CHAPTER VII

THE PUMP OF CTESIBIUS

1. Next I must tell about the machine of Ctesibius, which raises water
to a height. It is made of bronze, and has at the bottom a pair of
cylinders set a little way apart, and there is a pipe connected with
each, the two running up, like the prongs of a fork, side