A Test of an Imaginary

8/9/2019 A Test of an Imaginary

1/7

A Test of an "Imaginary" Experiment of Galileo'sAuthor(s): James MacLachlan

Reviewed work(s):Source: Isis, Vol. 64, No. 3 (Sep., 1973), pp. 374-379Published by: The University of Chicago Press on behalf of The History of Science SocietyStable URL: http://www.jstor.org/stable/229724 .

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2/7


3/7

TEST OF AN "IMAGINARY"

GALILEAN EXPERIMEN E

375

that are widely

acknowledged

to

have played

an

important

role in the

development

of

certain ideas.4

Yet, contention

remains about which of Galileo's

other

experiments

were imaginary

and

which

were real. In fact,

some

of

Galileo's experiments

are

less

real

than

others.

On the

one

hand,

his claim that the period of a pendulum is entirely independent of

amplitude can

easily be demonstrated

not

to

be

true for

amplitudes

greater

than

30

degrees.5

On

the other hand,

although Koyre

considered

Galileo's

inclined-plane

experiment to be "completely worthless,"6

Thomas Settle

has shown that

it

was

likely

a

real

experiment.

He

tested the

experimentaccording

to Galileo's

description

and

found

there to be no difficulty in supposing

Galileo to have had

sufficient resources to attain

the

results

he

described.7

Did Galileo

engage deeply

in the direct interrogation

of

nature,

or was

he

more

con-

cerned

with

shifting science from

the shoulders of

Aristotle

to

those

of

Plato? Con-

tinuing disagreementover this issue should mean that the examination of particular

experimental claims by Galileo

can

contribute to

our

proper understanding

of

the

part played by experience

in

the development

of his

ideas. To that

end,

here

is

the

report

of a test of another Galilean experiment

identified

as

imaginary by

Koyre.8

II

In a passage in the First Day of

the Two New Sciences

Salviati describeda procedure

(see Fig. 1,

in

Sec.

V) involving

water

and

wine :9

. . . I filled with watera glassball that had an openingas narrowas a straw stem ...

and

turned

t

over with its

mouth downward.However,neither

he

water,althoughvery

heavy

and suited

to

falling

throughair,

nor

the air, althoughvery

light

and much

in-

clined

to rise in

water, will agree,

the

former o

falling

out

of

the hole

[of

the

ball],the

latter

o

risingupon entering therein];

ut

remain,

both

of

them,

stubborn

nd

perverse

[in

their

places].

On the contrary,as soon

as

we

shall

present

o

that

hole

a

vessel con-

taining

red

wine, which is only imperceptibly

ess

heavy

than

water,

we shall

see

it

immediately

ise slowly in red streaks hrough

the

water;

and the

water,

with

the same

slowness,

descend

through

the wine,

without

in

the

least

mixing

together,

until

finally

the

ball

would be

completely

ull of

wine,

and

all the

water

would

fall

to

the

bottom

of

the vessel. Now, what

should one

say,

and

what

arguments

hould

be

appealedto,

exceptthat there s betweenwaterand airan incompatibilityhat I do not understand,

but

which,

perhaps....10

4T. S. Kuhn, "A Function for Thought

Experiments," in L'aventure de l'esprit. Melanges

Alexandre Koyre (Paris:Hermann, 1964), pp.

307-334.

5

Le Opere

di

Galileo Galilei, Edizione

Nazionale, ed. A. Favaro (Florence:Tipografia

Barbera, 1890-1909), Vol. VIII, p.

139;

in

Two New Sciences, trans. H. Crew and A. de

Salvio (New York: Macmillan, 1914and numer-

ous

reprints), pp.

95-96;

cf.

Marin

Mersenne,

Les nouvelles pensees de Galil6e (Paris: Guenon,

1639),pp. 72-73.

6

Koyre, Metaphysics

and

Measurement, p.

94.

7

T. Settle, "An Experiment n the History of

Science," Science, 1961, 133:19-23.

8

A.

Koyre,

"Le

De

Motu

Gravium

de

Galilee,

De

l'experienceimaginaire

et de

son

abus,"

Revued'Histoire

des Sciences, 1960, 13:197-245.

Reprinted n Koyre, Metaphysicsand

Measure-

ment, pp. 44-88;

trans.

by

R. E.

W. Maddison,

with the title "Galileo's Treatise De

Motu

Gravium,The Use and

Abuse

of

Imaginary

Experiment."

9

Galileo, Opere, Vol. VIII, pp. 115-116;

Two

New

Sciences

Crew

and de

Salvio),p. 71.

10

To provide

a

close

comparison

with

the

comments

of Prof.

Koyre,

this

passage

has

been

translated

from

his

French translation

in

Rev. Hist. Sci.,

pp. 240-241.

The

Galileo

passage

in

Maddison's

translation

(Koyre,

Metaphysics

and

Measurement, . 83)

was

made

directly

rom

the

Italian.


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4/7


5/7

TEST OF AN

"IMAGINARY" GALILEAN EXPERIMENT

377

directly to

the surface of

the

wine. (2) Several varieties

of red

wine were used (though,

fortunately,

none

from the seventeenth century). (3) The bottle and goblet were

replaced with a Florence flask

and a beaker, using again a

piece of drinking straw for

the outlet

from

the flask.

In

all

variations essentially

the

same

results

were

observed,

with the

clear

layer

of

water

beneath the wine occupying from 40 to 60 per cent of the

original

wine volume.

In some

cases the water

in

the bottle became

mixed

up

with wine

more

quickly

than in

the

first trial, but the pronounced clear layer beneath the wine

always appeared

in

the lower vessel. With a

tube

narrower

than

the drinking straw

the

exchange

of

liquids

was

minimal,

and

the

trial

was terminated.

IV

Professor Koyre proposed

another variation:

Resultsmorenearly n agreementwith Salviati's ssertionwould be obtainedby having

two

openings, nstead

of

one,

in the

glass flask,

and

fitting

a

straw,

or

a narrow

ube, to

each

in

such a

manner hat one (A)

is directed o the interiorof

the flask,

and the

other

(B) to the exterior.We should then see a streak of wine

streaming rom the tube A

towards

he top

of

the

flask,

and

a

streak

of

water

streaming

o the bottom

of

the

vessel,

with

the result

that the wine

would collect at the

top,

and

the

water

at

the

bottom.

Unfortunately,even

in this

case, there would

be

mixing. Furthermore,Salviati pro-

vides

one

orificeonly

to his

flask,

not

two;

nor does he

provideany straw.13

Koyr6

seems

to have

thought

that the water

and

wine

needed

separate

tubes

in

order

to

be

exchanged with

less

mixing (see Fig. 2).

As a

matter

of

fact, using

two tubes

changes the experiment

drastically and quite destroys

the

contrast

Galileo had estab-

lished. For, if a flask of water

with two tubes in its mouth

is

inverted

in

air,

then

water

and air will

exchange places

much more

readily than

the

water

and wine

do. Yet

Galileo had

appealed

to

this

experiment

to

demonstrate an

incompatibility

between

water and air that

does

not exist

between

water and wine.

Besides,

I have shown that a

single opening

is

sufficient,

and

even that

no

straw

is

needed to conduct the

liquids.

Nevertheless,

the

two-tube experiment

was worth

trying.

It

was

rather

more

difficult

to

perform

because one

of

the tubes

had

to

be

clamped

to

prevent

the

water

from

flowing

out

(and

air

in)

while

the bottle was

being

inverted and the

tubes

submerged

beneath

the wine. Once that

was done, the

transfer

of

liquids

followed essentially the

same course

as before-except that the whole operation

occurred much

more

quickly.

Only

about 15 minutes

were

needed to

arrive at

a result that had taken 90 minutes

using

only

one

tube

(or

none,

when the

narrow mouth of the bottle

was

applied

to

the wine

surface).

v

For

the

experiment

to

proceed

as Galileo

described

it,

the water must

fall

through

the

wine

gently enough

that

no turbulence

is

produced

to

cause the water and

wine to

mix. At the same time, the water must traverseits path in a time short enough to fore-

stall

mixing by

molecular diffusion.

The

results

of

my

test and an

elementary applica-

tion

of

hydrodynamic principles

suggest

that

the

size of

the hole

in

the

globe

of

water

is a

critical factor

in

determining

the

nature of

the downward flow

of

water.

'3

Koyre,Metaphysics

ndMeasurement, .

84n; cf.

Rev.Hist. Sci., p.

241n.




6/7

378

JAMES

MACLACHLAN

water

~ ~ ~ ~ ~

wae

Figure

1.

Galileo's

experiment.

Figure

2.

Koyr6's experiment.

At the water-winenterface

in

the

hole)

there sa

tendency

o

instability

s a resultof

the

greater

density

of the water.Fora small

enough

hole

(perhaps millimeters)

ur-

face-tensi'on

ffects

may

be sufficiento inhibit he flow of waterand wine. For a

large

enough

hole

(upward

f 10

millimeters)

he

instability

s so

great

hat the flow of water

is

turbulent

nonlaminar)

nd

considerable

mixing

will occur.114

vidently

Galileo's

"1straw

tem"

and

my

4-

to 6-millimeter

iameters re

optimal

or

the

occurrence f

a

smooth,

aminar

low

of

the

water.

For a hole

of

the

optimum

size, then,

the

instability

at the water-winenterface

results

n the

water

"dropping"hrough

he

hole,

displacing

he

wine,

and

settingup

a

convective

low in the wine.The wine is extruded

nto the

watercontainer

above,

n a

thin

jet.

The

"wafting"

of the wine

upward hrough

he bottle

suggests

a

degree

of

instability esulting

roma

higher

inear

speed

han

that of the

water

streaming

own-

ward,

ndicating

hat the water

occupies

a

larger

raction

of

the

hole than

the

wine.A

14

For a hole that large, of course, water

will

flow out quite readily

when the bottle is held

mouth downward n air, so that theconditions of

the Galileanexperiment re againnot

realized.




7/7

TEST OF AN

"IMAGINARY" GALILEAN EXPERIMENT 379

more detailed

account

of

this phenomenon would require a rather complex

analysis

in

hydrodynamics, which has not yet been undertaken. 5

When a

narrow-mouthed vessel full of

water

is

inverted

in air

the water

does

not

fall

out. Galileo attributed this

phenomenon to an "incompatibility" between water and

air. Now, when the same vessel

is

inverted with its mouth beneath the surface of wine,

the

two

liquids exchange places,

not

exhibiting

that kind of

incompatibility.

However,

since

the manner of exchange of

the two liquids made no contribution to Galileo's

argument, I believe that he would

have been satisfied if the water and wine had

merely

interpenetrated gradually. But

they did not; and Galileo's detailed

description of the

striking behavior of the wine and water convinces me that he did indeed see

what I

have seen. However imaginary this experiment may have been for

Koyre,

it

was cer-

tainly

a

real

experiment

for

Galileo.

15

1

am grateful to Prof. C. 0. Hines of the

University

of

Toronto

and Dr. W. R. Peltier

of

the

University

of Coloradofor

discussionsof the

hydrodynamic

principles involved in this ex-

periment.This researchhas also benefited rom

the kind encouragement f

Prof. StillmanDrake

of

the

Universityof Toronto.

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