n place of the sun-dial, to mark the hours. But as
the length of the hour varied according to the season of the year,
various arrangements, of which we have no clear account, were necessary
to obviate this and other defects. For instance, the flow of water
varied with the temperature and pressure of the air, and secondly, the
rate of flow became less as the vessel emptied itself. The latter defect
was remedied by keeping the level of the water in the clepsydra uniform,
the volume of that discharged being noted. Plato is said to have
invented a complicated clepsydra to indicate the hours of the night as
well as of the day. In the clepsydra or hydraulic clock of Ctesibius of
Alexandria, made about 135 B.C., the movement of water-wheels caused the
gradual rise of a little figure, which pointed out the hours with a
little stick on an index attached to the machine. The clepsydra is said
to have been known to the Egyptians. There was one in the Tower of the
Winds at Athens; and the turret on the south side of the tower is
supposed to have contained the cistern which supplied the water.

See Marquardt, _Das Privatleben der Roemer_, i. (2nd ed., 1886), p.
792; G. Bilfinger, _Die Zeitmesser der antiken Voelker_ (1886), and
_Die antiken Stundenangaben_ (1888).

CLERESTORY, or CLEARSTORY (Ital. _chiaro piano_, Fr. _clairevoie_,
_claire etage_, Ger. _Lichtgaden_), in architecture, the upper storey of
the nave of a church, the walls of which rise above the aisles and are
pierced with windows ("clere" being simply "clear," in the sense of
"lighted"). Sometimes these windows are very small, being mere
quatrefoils or spherical triangles. In large buildings, however, they
are important objects, both for beauty and utility. The windows of the
clerestories of Norman work, even in large churches, are of less
importance than in the later styles. In Early English they became
larger; and in the Decorated they are more important still, being
lengthened as the triforium diminishes. In Perpendicular work the latter
often disappears altogether, and in many later churches, as at Taunton,
and many churches in Norfolk and Suffolk, the clerestories are close
ranges of windows. The term is equally applicable to the Egyptian
temples, where the lighting of the hall of columns was obtained over the
stone roofs of the adjoining aisles, through slits pierced in vertical
slabs of stone. The Romans also in their baths and palaces employed the
sa