III
The idea of time in antiquity differed from ours not
only because it was thought to be cyclical but also
because the lack of reliable mechanical clocks pre-
vented its accurate measurement. This impeded the
development of the modern metrical concept of time.
Moreover, the scale of “hours” was not uniform. In-
deed, our present system of dividing day and night
together into twenty-four hours of equal length was
not employed in civil life until the fourteenth century
A.D., although it had already been used by astronomers.
Previously, it was the general custom to divide the
periods of light and darkness into an equal number of
“temporal hours” (horae temporales, as they were
called by the Romans). The number was usually twelve.
Consequently, the length of an hour varied according
to the time of year and also, except at the equinoxes,
a daylight hour was not equal to a nocturnal hour.
Strange as this mode of reckoning time may now seem,
we must remember that most human activities took
place in the hours of daylight and also that early civili-
zations were in latitudes where the period from sunrise
to sunset varies far less than in more northerly parts.
For their standard hours, the astronomers took “equi-
noctial hours” (horae equinoctales). These were the
same as the temporal hours at the date of the spring
equinox.
The only mechanical time-recorders in antiquity
were water clocks, but until the fourteenth century
A.D. the most reliable way to tell the time was by means
of a sundial. Both types of clock were used by the
Egyptians. Later they were introduced into Greece and
eventually became widespread in the Roman Empire.
Vitruvius, writing about 30 B.C., described more than
a dozen different types of sundial. He also described
a number of “clepsydrae” or water clocks. To obtain
a uniform flow of water they were designed so as to
keep the pressure head constant. In order to indicate
“temporal” hours, either the rate of flow or the scale
of hours had to be varied according to the time of year.
The result was that many of the ancient water clocks
were instruments of considerable complexity.
The earliest known attempt to produce mechanically
a periodic standard of time is a device illustrated in
a Chinese text written by Su Sung in A.D. 1092. It was
powered by a waterwheel which advanced in a step-
by-step motion, water being poured into a series of
cups which emptied (or escaped) every quarter of an
hour, when the weight of the water in the cup was
sufficient to tilt a steelyard. The mechanism was then
unlocked until the arrival of the next cup below the
water stream when it was locked again. An astronomi-
cal check on timekeeping was made by a sighting tube
pointed to a selected star. Since the timekeeping was
governed mainly by the flow of water rather than by
the escapement action itself, this device may be re-
garded as a link between the timekeeping properties
of a steady flow of liquid and those of mechanically
produced oscillations.
The fundamental distinction between water clocks
and mechanical clocks, in the strict sense of the term,
is that the former involve a continuous process (the
flow of water through an orifice) whereas the latter
are governed by a mechanical motion which continu-
ally repeats itself. The mechanical clock, in this sense,
appears to have been a European invention of the late
thirteenth or early fourteenth century. The first clocks
of this type were public striking clocks, the earliest,
as far as we know, being set up at Milan in 1309. The
type of motion employed in these clocks, known as
the “verge” escapement—probably from the Latin
virga, a rod or twig—was an ingenious device in which
a heavy bar pivoted near its center was pushed first
one way and then the other by a toothed wheel driven
by a weight suspended from a drum. The wheel ad-
vanced by the space of one tooth for each to and fro
oscillation of the bar. Since the bar had no natural
period of its own, the rate of the clock depended on
the driving weight, but was also affected by variations
of friction in the driving mechanism. Consequently, the
accuracy of these clocks was low and they could not
be relied on to keep time more closely than to about
a quarter of an hour a day at best. An error of an hour
was not unusual. Until the middle of the seventeenth
century mechanical clocks had only one hand and the
dial was divided only into hours and quarters.
The word “clock” is etymologically related to the
French word cloche, meaning a bell. Bells played a
prominent part in medieval life and mechanisms for
ringing them, made of toothed wheels and oscillating
levers, may have helped to prepare for the invention
of mechanical clocks. Indeed, some early clocks were
essentially mechanisms for striking the hours.
Music provides another instance of the growing
importance of temporal concepts in the Middle Ages.
Early medieval music was all plain chant in which
notes had fluid time-values. Mensural music in which
the duration of notes had an exact ratio among them-
selves appears to have been an Islamic invention. It
was introduced into Europe about the twelfth century.
About this time there appeared in Europe the system
of notation in which the exact time-value of a note
is indicated by a lozenge on a pole.
