Foucault took advantage of the height of
the dome, nearly three hundred feet, and had a heavy weight suspended by
a wire from its loftiest point, forming an immense pendulum,--the
longest, I suppose, ever constructed. Now a moving body tends to keep
its original plane of movement, and so the great pendulum, being set
swinging north and south, tended to keep on in the same direction. But
the earth was moving under it, and as it rolled from west to east the
plane running through the north and south poles was every instant
changing. Thus the pendulum appeared to change its direction, and its
deviation was shown on a graduated arc, or by the marks it left in a
little heap of sand which it touched as it swung. This experiment on the
great scale has since been repeated on the small scale by the aid of
other contrivances.
My thoughts wandered back, naturally enough, to Galileo in the Cathedral
at Pisa. It was the swinging of the suspended lamp in that edifice which
set his mind working on the laws which govern the action of the
pendulum. While he was meditating on this physical problem, the priest
may have been holding forth on the dangers of meddling with matters
settled by Holy Church, who stood ready to enforce her edicts by the
logic of the rack and the fagot.
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