1 The Birth of Science - MSRI

4.2 Instrumentation 89
be kept in place at the water level by means of screws. Each plate had a
peephole for alignment.
Instruments such as these are incompatible with the widespread belief
in a constant rate of progress and in the primitive state of classical technology.
Those historians of technology who, imbued with these beliefs,
nonetheless studied Heron’s dioptra have become aware of the existence
of an error; however, they generally attribute the error to Heron himself,
for failing to keep his designs in line with the age in which he happened page 126
to live. For example, in the authoritative History of technology edited by
Singer et al., we read:
Heron’s dioptra remains unique, without past and without future: a
fine but premature invention whose complexity exceeded the technical
resources of its time. 15
In fact by Heron’s time the dioptra was already centuries old, as can be
seen just by reading the Almagest (see note 41 on page 243). As for the
future of the dioptra, it contained if nothing else the theodolite, born in
the sixteenth century of the study of Heron’s work.
Timepieces. The main Hellenistic instrument for measuring time was the
water clock. Its ancestor was the water clepsydra of Pharaoh-era Egypt,
which was simply a container with an orifice in the bottom. The time
elapsed from the filling of the container could be read off from the level of
the water against a scale drawn on the interior. A clepsydra of this type,
from about 1400 B.C., was found in Karnak. 16
The Egyptian clepsydra served to give an idea of the passage of time,
particularly at night, but it cannot be said to have been a true measuring
instrument, for two reasons. First, the rate at which the water flows out
depends on the pressure and therefore decreases as more water escapes.
Second, the size of the hole cannot be regarded as constant except in the
short term, because of corrosion and accretions that tend to constrict it.
The Egyptians had partly overcome the first difficulty — but not the second
— by using containers shaped as truncated cones rather than cylinders;
as far as we know this was a purely qualitative correction. A further
complication was caused by the unit of time used then, the hour, which
changed from day to day and between day and night, being defined as
one-twelfth the time between sunrise and sunset and vice versa.
Classical Greece made no essential improvements to the Egyptian clep- page 127
15 [Price: Instruments], p. 612.
16 See [Borchard], pp. 6–7.
Revision: 1.14 Date: 2002/10/24 04:25:47