Descartes: A Biography

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 Descartes: A Biography

P ́erier, asking him to conduct what became one of the most famous exper-

iments of the seventeenth century. Poor weather conditions and P ́erier’s

professional duties delayed the planned experiment for almost ten months.

However, in September,Perier set off to test the theory that a column ́

of mercury is supported in a Torricelli tube by the weight of the column

of air that presses on the surface of the mercury. The experiment worked

perfectly. P ́erier set out in the morning, with five reliable witnesses, to

measure the height of mercury in an inverted glass tube at the bottom

of the mountain called the Puy-de-Dome and at various intervals as theyˆ

climbed to the top. They brought two similar tubes, filled with mercury

and inverted in the usual way. The height of mercury was equal in both

tubes, and it was measured and recorded. Perier left one tube in posi- ́

tion at the bottom of the mountain, in the care of a Minim friar who was

charged with watching it during the day and recording any variations in

the height of the column of mercury. Meanwhile, P ́erier and his assistants

climbed the mountain and took measurements at various places until they

reached the top. At that point, the mercury had dropped ‘three inches and

oneand a half lines’. This was the anticipated result, which so excited the

participants that they repeated the measurement a number of times in var-

ious weather conditions throughout the day. The team of observers then

descended the mountain until they reached the friary garden, where Father

Chastin reported that his column of mercury ‘had remained unchanged

all day, despite the fact that the weather was very changeable, sometimes

calm, sometimes rainy, sometimes very foggy and sometimes windy’.To

the delight of all concerned, the other column of mercury, which had been

carried up and down the Puy-de-Dome, returned to its original reading.ˆ

The results were reported in a pamphlet entitledDescription of the Great

Experiment on the Equilibrium of Liquidsin.

The conclusion for which Pascal argued was that the mercury is sup-

ported to a height of about thirty inches by the column of air above it

that presses down on the surface of the mercury. Since the weight of that

column of air should be reduced as one climbs a high mountain, the height

of the mercury in the tube should decrease proportionately. By contrast,

if nature’s abhorrence of a vacuum were the correct explanation, there

would be no reason to think that nature would abhor a vacuum less on

top of a mountain than lower down, and there should therefore be no cor-

responding change in the column of mercury. Descartes agreed that the

phenomenon should not be explained by reference to nature’s apparent