Classic History Books

William J. Claxton - The Mastery of the Air

authorities as directors of this campaign of concealment. In this connection it is interesting to note that
both Admiralty and War Office took measures to record the pictorial side of the Great War. Special

commissions were given to a notable band of artists working in their different "lines". An abiding record

of the great struggle will be afforded by the black-and-white work of Muirhead Bone, James M'Bey, and

Charles Pears; the portraits, landscapes, and seascapes of Sir John Lavery, Philip Connard, Norman

Wilkinson, and Augustus John, who received his commission from the Canadian Government.


CHAPTER XL. The Atmosphere and the Barometer

For the discovery of how to find the atmospheric pressure we are indebted to an Italian named Torricelli,
a pupil of Galileo, who carried out numerous experiments on the atmosphere toward the close of the

sixteenth century.

Torricelli argued that, as air is a fluid, if it had weight it could be made to balance another fluid of known
weight. In his experiments he found that if a glass tube about 3 feet in length, open at one end only, and

filled with mercury, were placed vertically with the open end submerged in a cup of mercury, some of

the mercury in the tube descended into the cup, leaving a column of mercury about 30 inches in height in

the tube. From this it was deduced that the pressure of air on the surface of the mercury in the cup forced

it up the tube to the height Of 30 inches, and this was so because the weight of a column of air from the

cup to the top of the atmosphere was only equal to that of a column of mercury of the same base and 30

inches high.

Torricelli's experiment can be easily repeated. Take a glass tube about 3 feet long, closed at one end and
open at the other; fill it as full as possible with mercury. Then close the open end with the thumb, and

invert the tube in a basin of mercury so that the open end dips beneath the surface. The mercury in the

tube will be found to fall a short distance, and if the height of the column from the surface of the mercury

in the basin be measured you will find it will be about 30 inches. As the tube is closed at the top there is

no downward pressure of air at that point, and the space above the mercury in the tube is quite empty: it

forms a VACUUM. This vacuum is generally known as the TORRICELLIAN VACUUM, after the name

of its discoverer.

Suppose, now, a hole be bored through the top of the tube above the column of mercury, the mercury will
immediately fall in the tube until it stands at the same level as the mercury in the basin, because the

upward pressure of air through the liquid in the basin would be counterbalanced by the downward

pressure of the air at the top, and the mercury would fall by its own weight.

A few years later Professor Boyle proposed to use the instrument to measure the height of mountains. He
argued that, since the pressure of the atmosphere balanced a column of mercury 30 inches high, it

followed that if one could find the weight of the mercury column one would also find the weight of a

column of air standing on a base of the same size, and stretching away indefinitely into space. It was

found that a column of mercury in a tube having a sectional area of 1 square inch, and a height of 30

inches, weighed 15 pounds; therefore the weight of the atmosphere, or air pressure, at sea-level is about

15 pounds to the square inch. The ordinary mercury barometer is essentially a Torricellian tube graduated

so that the varying heights of the mercury column can be used as a measure of the varying atmospheric

pressure due to change of weather or due to alteration of altitude. If we take a mercury barometer up a

hill we will observe that the mercury falls. The weight of atmosphere being less as we ascend, the

column of mercury supported becomes smaller.

Although the atmosphere has been proved to be over 200 miles high, it has by no means the same density


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