ts of the months.
Behind these rods there is a drum, on which is drawn and painted the
firmament with the circle of the signs. In drawing the figures of the
twelve celestial signs, one is represented larger and the next smaller,
proceeding from the centre. Into the back of the drum, in the middle, a
revolving axis is inserted, and round that axis is wound a flexible
bronze chain, at one end of which hangs the "cork" which is raised by
the water, and at the other a counterpoise of sand, equal in weight to
the "cork."

9. Hence, the sand sinks as the "cork" is raised by the water, and in
sinking turns the axis, and the axis the drum. The revolution of this
drum causes sometimes a larger and sometimes a smaller portion of the
circle of the signs to indicate, during the revolutions, the proper
length of the hours corresponding to their seasons. For in every one of
the signs there are as many holes as the corresponding month has days,
and a boss, which seems to be holding the representation of the sun on a
dial, designates the spaces for the hours. This, as it is carried from
hole to hole, completes the circuit of a full month.

10. Hence, just as the sun during his passage through the constellations
makes the days and hours longer or shorter, so the boss on a dial,
moving from point to point in a direction contrary to that of the
revolution of the drum in the middle, is carried day by day sometimes
over wider and sometimes over narrower spaces, giving a representation
of the hours and days within the limits of each month.

To manage the water so that it may flow regularly, we must proceed as
follows.

11. Inside, behind the face of the dial, place a reservoir, and let the
water run down into it through a pipe, and let it have a hole at the
bottom. Fastened to it is a bronze drum with an opening through which
the water flows into it from the reservoir. Enclosed in this drum there
is a smaller one, the two being perfectly jointed together by tenon and
socket, in such a way that the smaller drum revolves closely but easily
in the larger, like a stopcock.

12. On the lip of the larger drum there are three hundred and sixty-five
points, marked off at equal intervals. The rim of the smaller one has a
tongue fixed on its circumference, with the tip directed towards those
points; and also in this rim is a small opening, through which water
runs into the drum and keeps the works going. The figures of the
celestial signs being