1 The Birth of Science - MSRI

11.4 A Late Disciple of Archimedes 311
based on the cohesion of water particles, 80 restates that of the Pneumatics
of Philo of Byzantium for the drinking straw and the syphon. 81
Regarding catoptrics, the experimental facts that most interest Galileo
and his students are those regarding burning mirrors: on the first day
of the Dialogues, Sagredo says he has seen a spherical mirror melt lead,
and he imagines from this the enormous power of the Archimedean mirrors,
which he knows were parabolic. 82 The Dialogues came out only six
years after Bonaventura Cavalieri’s important treatise on The burning mirror,
82a which treated many applications of the classical theory of conics,
including parabolic mirrors (using newly relearned knowledge about the
parabola’s focal property), and even demonstrated that a body under the
action of gravity follows a parabolic trajectory.
Skipping over Galileo’s prominent work in observational astronomy
(because, as we know, many don’t grant such things the status of “experimental
science”), we turn to his interest, in the next few years, in other
Hellenistic experimental subjects such as statics, aerostatics and thermology.
We single out his studies on the expansion of heated gases (a subject
that around that time also interested Della Porta and van Helmont 83 ), page 384
which led to the reconstruction of experiments and devices described by
Philo and Heron, opening the way to the construction of modern thermometers.
84
Galileo’s most famous experiment involved the motion of a body along
an inclined plane. Let’s see now what he says about the crucial matter of
measuring the time of descent:
For the measurement of time, we employed a large vessel of water
placed in an elevated position; to the bottom of this vessel was soldered
a pipe of small diameter giving a thin jet of water, which we
collected in a small glass during the time of each descent, whether
for the whole length of the channel or for a part of its length; the
80
Galileo Galilei, Discorsi e dimostrazioni. . . , day 1, at Sagredo’s 12th turn = [Galileo: Opere],
vol. VIII, pp. 64–65.
81
Philo of Byzantium, Pneumatica, iii = [Philo/Prager], 81, 129–130. But the cohesion argument
is not in Heron’s Pneumatica, and moreover Empedocles already knew that air pressure can overcome
the weight of water, as happens with the clepsydra (note 77 on page 67), the syringe and
the syphon. So perhaps Galileo was led into error by Philo’s text, which survived to modern times
only in very corrupt Latin and Arabic translations. (The explanation chosen by Galileo is clearer in
the Latin text, which seems to be the farther from the original; see [Philo/Prager], p. 81.)
82
Galileo Galilei, Discorsi e dimostrazioni. . . , day 1, at Sagredo’s 25th turn = [Galileo: Opere],
vol. VIII, p. 86.
82a
See note 85 on page 104.
83
See note 188 on page 278.
84
Galileo’s thermoscope, essentially a gas-expansion thermometer, is the device described by Philo
of Byzantium (see page 277).
Revision: 1.11 Date: 2003/01/06 07:48:20