with
polygonal blocks of lava.

CLOCK. The measurement of time has always been based on the revolution
of the celestial bodies, and the period of the apparent revolution of
the sun, i.e. the interval between two consecutive crossings of a
meridian, has been the usual standard for a day. By the Egyptians the
day was divided into 24 hours of equal length. The Greeks adopted a
different system, dividing the day, i.e. the period from sunrise to
sunset, into 12 hours, and also the night. Whence it followed that it
was only at two periods in the year that the length of the hours during
the day and night were uniform (see CALENDAR). In consequence, those who
adopted the Greek system were obliged to furnish their water-clocks (see
CLEPSYDRA) with a compensating device so that the equal hours measured
by those clocks should be rendered unequal, according to the exigencies
of the season. The hours were divided into minutes and seconds, a system
derived from the sexagesimal notation which prevailed before the decimal
system was finally adopted. Our mode of computing time, and our angular
measure, are the only relics of this obsolete system.

The simplest measure of time is the revolution of the earth round its
axis, which so far as we know is uniform, perfectly regular, and has not
varied in speed during any period of human observation. The time of such
a revolution is called a sidereal day, and is divided into hours,
minutes and seconds. The period of rotation of the earth is practically
measured by observations of the fixed stars (see TIME), the period
between two successive transits of the same star across a meridian
constituting the sidereal day. But as the axis of the earth slowly
revolves round in a cone, whereby the phenomenon known as the precession
of the equinoxes is produced, it follows that the astronomical sidereal
day is not the true period of the earth's rotation on its axis, but
varies from it by less than a twenty millionth part, a fraction so small
as to be inappreciable. But the civil day depends not on the revolution
of the earth with regard to the stars, but on its revolution as compared
with the position of the sun. Therefore each civil day is on the average
longer than a sidereal one by nearly four minutes, or, to be exact, each
sidereal day is to an average civil day as .99727 to 1, and the sidereal
hour, minute and second are also shorter in like proportion. Hence a
sidereal clock has a shorter, qui