The Science Book

47

See also: Isaac Newton 62–69 ■ John Dalton 112–13 ■ Robert FitzRoy 150–55

Barometers
In Italy, the mathematician Gasparo
Berti performed experiments
designed to figure out why a
suction pump could not raise water
more than 33 ft (10 m) high. Berti
took a long tube, sealed it at one
end and filled it with water. He then
inverted the tube with its mouth in
a tub of water. The level of water
in the tube fell until the column
was about 30 ft (10 m) high. In
1642, fellow Italian Evangelista
Torricelli, hearing of Berti’s work,
constructed a similar apparatus
but used mercury instead of water.
Mercury is more than 13 times
denser than water, so his column
of liquid was only about 30 in
(76 cm) high. Torricelli’s explanation
for this was that the weight of the
air above the mercury in the dish
was pressing down on it, and that
this balanced the weight of the
mercury inside the column.

He said that the space in the tube

above the mercury was a vacuum.

This is explained today in terms

of pressure (force on a certain area),

but the basic idea is the same.

Torricelli had invented the first

mercury barometer.

French scientist Blaise Pascal

heard of Torricelli’s barometer

in 1646, prompting him to start

some experiments of his own.

One of these, performed by his

brother-in-law Florin Périer, was

SCIENTIFIC REVOLUTION

Blaise Pascal’s experiments with
barometers showed how air pressure
varied with altitude. In addition to
physics, Pascal also made significant
contributions to mathematics.

We live submerged at the

bottom of an ocean of

the element air, that by

unquestioned experiments

is known to have weight.

Evangelista Torricelli

to demonstrate that air pressure

changed depending on altitude.

One barometer was set up on

the grounds of a monastery in

Clermont, and observed by a monk

during the day. Périer carried the

other to the top of Puy de Dôme,

about 3,200 ft (1,000 m) above the

town. The column of mercury was

more than 3 in (8 cm) shorter at

the top of the mountain than in the

monastery garden. Since there is

less air above a mountain than

there is above the valley below

it, this showed that it was indeed

the weight of the air that held the

liquid in the tubes of mercury or

water. For this, and other work,

the modern unit of pressure is

named after Pascal.

Air pumps

The next important breakthrough

was made by Prussian scientist

Otto von Guericke, who made a

pump that was capable of pumping

some of the air out of a container.

He performed his most famous ❯❯

The barometer

invented by

Evangelista Torricelli

used a column of

mercury to measure

air pressure. Torricelli

correctly reasoned

that it was the air

pressing down on

the mercury in the

cistern that balanced

the column of

mercury in the tube.

Mercury

Pressure of

mercury column

Scale

Cistern (dish)

Pressure of

atmosphere

Torricellian vaccum

Tube