102
THE ATTRACTION
OF MOUNTAINS
NEVIL MASKELYNE (1732–1811)
I
n the 17th century, Isaac
Newton had suggested
methods for “weighing the
Earth”—or calculating Earth’s
density. One of these involved
measuring the angle of a plumb
line on each side of a mountain to
find out how far the gravitational
attraction of the mountain pulled
it from the vertical. This deviation
could be measured by comparing
the plumb line to a vertical
calculated using astronomical
methods. If the density and
volume of the mountain could be
ascertained, then, by extension,
so could the density of Earth.However, Newton himself
dismissed the idea because
he thought the deviation would
be too small to be measured
with the instruments of the day.
In 1738, Pierre Bouguer, a
French astronomer, attempted
the experiment on the slopes of
Chimborazo in Ecuador. Weather
and altitude caused problems,
however, and Bouguer did not think
his measurements were accurate.
In 1772, Nevil Maskelyne
proposed to the Royal Society in
London that the experiment could
be conducted in Britain. The
Society agreed, and sent a surveyorIN CONTEXT
BRANCH
Earth science and physicsBEFORE
1687 Isaac Newton publishes
the Principia, in which he
suggests experiments for
calculating Earth’s density.1692 In an effort to explain
Earth’s magnetic field,
Edmond Halley suggests that
the planet consists of three
concentric hollow spheres.1738 Pierre Bouguer attempts
Newton’s experiment, without
success, on Chimborazo, a
volcano in Ecuador.AFTER
1798 Henry Cavendish
uses a different method to
calculate the density of
Earth, and finds it to be
340 lb/ft^3 (5,448 kg/m^3 ).1854 George Airy figures
out Earth’s density using
pendulums in a mine.Measuring the deviation
should allow calculation
of Earth’s mass.The gravitational mass
of a mountain should
attract a plumb bob.The plumb line
will hang at an angle that
depends on the relative
density of the mountain
and Earth.