on the lip of the larger drum, and this drum being
motionless, let the sign Cancer be drawn at the top, with Capricornus
perpendicular to it at the bottom, Libra at the spectator's right, Aries
at his left, and let the other signs be given places between them as
they are seen in the heavens.

13. Hence, when the sun is in Capricornus, the tongue on the rim touches
every day one of the points in Capricornus on the lip of the larger
drum, and is perpendicular to the strong pressure of the running water.
So the water is quickly driven through the opening in the rim to the
inside of the vessel, which, receiving it and soon becoming full,
shortens and diminishes the length of the days and hours. But when,
owing to the daily revolution of the smaller drum, its tongue reaches
the points in Aquarius, the opening will no longer be perpendicular, and
the water must give up its vigorous flow and run in a slower stream.
Thus, the less the velocity with which the vessel receives the water,
the more the length of the days is increased.

14. Then the opening in the rim passes from point to point in Aquarius
and Pisces, as though going upstairs, and when it reaches the end of the
first eighth of Aries, the fall of the water is of medium strength,
indicating the equinoctial hours. From Aries the opening passes, with
the revolution of the drum, through Taurus and Gemini to the highest
point at the end of the first eighth of Cancer, and when it reaches that
point, the power diminishes, and hence, with the slower flow, its delay
lengthens the days in the sign Cancer, producing the hours of the summer
solstice. From Cancer it begins to decline, and during its return it
passes through Leo and Virgo to the points at the end of the first
eighth of Libra, gradually shortening and diminishing the length of the
hours, until it comes to the points in Libra, where it makes the hours
equinoctial once more.

15. Finally, the opening comes down more rapidly through Scorpio and
Sagittarius, and on its return from its revolution to the end of the
first eighth of Capricornus, the velocity of the stream renews once more
the short hours of the winter solstice.

The rules and forms of construction employed in designing dials have now
been described as well as I could. It remains to give an account of
machines and their principles. In order to make my treatise on
architecture complete, I will begin to write on this subject in the
following book.