1 The Birth of Science - MSRI

88 4. Scientific Technology
Alexandria — by Arabs and Renaissance Europeans. They are not objects
thrust on us by nature or by logic, as many seem to believe, but cultural
products inherited from Hellenistic civilization.
4.2 Instrumentation
Archeological finds and surviving descriptions are so rare that we cannot
hope to know a good part of Hellenistic measuring instruments. But
we have enough information to form an idea of the prevailing qualitative
technological level. We will mention only two types of measuring instruments:
surveying instruments and timepieces.
Surveying instruments. The main surveying instrument of Pharaoh-era
Egypt had been the surveyor’s cross or groma, which consisted of two
perpendicular wood beams and allowed the recognition of right angles.
It was inherited essentially unchanged by classical Greece and by Rome.
The difference between prescientific technology and scientific technology page 125
is well illustrated by the comparison between the groma and a Hellenistic
instrument that can be used to the same end: the dioptra described by
Heron. 14 Here the perpendicular axes are drawn on a graduated disc; one
axis, at whose ends there are slits for looking through, is free to rotate on
the plane of the disc, around its center. The disc itself not only can rotate
freely about its center, but it is attached at right angles to the diameter of a
cogged semicircle made of brass, which can pivot about a horizontal axis;
an endless worm screw allows one to fine tune and secure the position.
The whole device described so far is attached by three pivots to a vertical
cylinder that can itself turn about its axis; here again fine tuning and fixing
depend on a worm, which meshes with a cog coaxial with the cylinder.
Heron mentions several uses for the dioptra, particularly in astronomical
measurements and in the surveying necessary for the excavation of underground
galleries. Unfortunately the description we have is incomplete
and we don’t know whether the missing part dealt with important components
of the instrument.
Heron also describes in the Dioptra a level, consisting of a horizontal
wooden pole, about two meters long, containing inside a tube with ends
turned up, to which two vertical glass tubes could be connected. When the
instrument was filled with water, the principle of communicating vessels
would ensure that the water would reach the same level in both glass
tubes. The tubes were endowed with small sliding brass plates that could
14 Heron, Dioptra = [Heron: OO], vol. 3, pp. 187–366.
Revision: 1.14 Date: 2002/10/24 04:25:47