Descartes: A Biography

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Once More into Battle 

of what was discussed during a three-hour conversation. Pascal’s sister,

Jacqueline, was unable to be present, and she failed to get a summary

from her brother later, because she and her assistants spent most of the

afternoon trying to bathe him. She subsequently asked Mersenne why

Descartes objected to Pascal’s account of air pressure. The unfortunate

Minim, who was still suffering the effects in his right arm of his surgeon’s

inept blood-letting, replied in hardly legible writing. Jacqueline’s account

is helpful about the domestic circumstances of the meeting between her

brother and Descartes. However, she understood very little about why they

disagreed, which was acknowledged when she discovered, later, that it was

Roberval rather than Descartes who opposed the theory of air pressure.

It had been known for some time that when a tube (closed at one end)

is filled with mercury, and when the tube is inverted with the open end

submerged below the surface of a dish of mercury, the mercury does not

flow out of the tube completely. Instead, it drops to a height of about thirty

inches, leaving an apparently empty space above the mercury. Scholastic

philosophers explained this phenomenon by saying that ‘nature abhors a

vacuum’; in order to avoid having a vacuum, the mercury rises in the tube to

the observed height. Descartes had been criticizing such explanations for

almost two decades. Any reference to nature’s ‘abhorrence’ either mistak-

enly attributed intentional states to nature when they belonged properly

only to people, or it camouflaged a failure to explain the phenomenon

byappeal to a metaphorical abhorrence. In addition to these objections

to pseudo-explanations, there was a significant amount of experimental

work being done at the time on this phenomenon. It was obviously impor-

tant for understanding the theoretical limits of even the most efficient

pumping devices, such as those used in the United Provinces for draining

marshes. The Florentine physicist Evangelista Torricelli (–) had

experimented with mercury tubes in. Once news of his work began

to spread around Europe, there were attempts to duplicate it by a number

of independent experimentalists, including Pierre Petit (who had earlier

criticized Descartes’Dioptrics) and an obscure Capuchin friar, Valeriano

Magno, who published his results in Warsaw.Thus when Pascal began to

do experiments with Torricelli tubes, he joined an ongoing experimental

and philosophical dialogue that had been initiated in Florence.

Pascal published a short booklet entitledNew Experiments Concerning

the Vacuumin October, after Descartes had returned from Paris to

Egmond.He also wrote one month later to his brother-in-law, Florin