The Science Book

103

Schiehallion was chosen as the site

for the experiment because it was

symmetrically shaped and isolated

(and therefore less affected by the

gravitational pull of other mountains).

See also: Isaac Newton 62–69 ■ Henry Cavendish 78–79 ■ John Michell 88–89

EXPANDING HORIZONS

to select an appropriate mountain.
He chose Schiehallion in Scotland,
and Maskelyne spent nearly four
months making observations from
both sides of the mountain.

The density of rocks
The orientation of the plumb line
compared to the stars should have
been different at the two stations
even without any gravitational
effects, because of the difference
in latitude. However, even when
this was accounted for, there was
still a difference of 11.6 seconds of

arc (just over 0.003 degrees).

Maskelyne used a survey of the

shape of the mountain and a

measurement of the density of its

rocks to figure out the mass of

Schiehallion. He was assuming

that the whole Earth had the same

density as Schiehallion, but the

deviation of the plumb lines

showed a measured value of less

than half of what he was expecting.

Maskelyne realized that the density

assumption was not correct—the

density of Earth was clearly much

greater than that of its surface

rocks, probably, he reasoned, due

to the planet having a metallic

core. The actual observed angle

was used to figure out that the

overall density of Earth is about

double that of Schiehallion’s rocks.

This result disproved one

theory of the time, advocated by

English astronomer Edmond Halley,

...the mean density of the

earth is at least double of that

at the surface...the density of

the internal parts of the earth

is much greater than near

the surface.

Nevil Maskelyne

Nevil Maskelyne Born in London in 1732, Nevil

Maskelyne became interested

in astronomy at school. After

graduating from Cambridge

University and being ordained a

priest, he became a member of

the Royal Society in 1758, and

was the Astronomer Royal from

1765 until his death.

In 1761, the Royal Society sent

Maskelyne to the Atlantic island of

St. Helena to observe the transit

of Venus. Measurements taken

as the planet passed across the

Sun’s disk allowed astronomers to

calculate the distance between

Earth and the Sun. He also spent

much time trying to solve the

problem of measuring longitude

while at sea—a major issue of

the day. His method involved

carefully measuring the

distance between the Moon

and a given star, and consulting

published tables.

Key works

1764 Astronomical Observations

Made at the Island of St Helena

1775 An Account of

Observations Made on

the Mountain Schehallien

for Finding its Attraction

that said Earth was hollow. It also

allowed the mass of Earth to be

extrapolated from its volume and

average density. Maskelyne’s value

for the overall density of Earth was

280 lb/ft^3 (4,500 kg/m^3 ). Compared

with today’s accepted value of

344 lb/ft^3 (5,515 kg/m^3 ), he had

figured out the density of Earth

with an error of less than 20 percent,

and in the process had proved

Newton's law of gravitation. ■