ving exhausted the air from a long glass tube in which a guinea piece
and a feather had been placed, he showed that in the vacuum thus formed
they fell with equal rapidity as often as the tube was reversed. From
his various experiments as to the pressure of the atmosphere he deduced
the law that the density and elasticity of the atmosphere are precisely
proportional to the compressing force (the law of Boyle and Mariotte).
He also ascertained that air existed in a state of mechanical
mixture with liquids, "existing between their particles in a state
of condensation." He made many other experiments, especially on
the collision of bodies, but his most important work was upon the
atmosphere.

But meanwhile another contemporary of Boyle and Mariotte was interesting
himself in the study of the atmosphere, and had made a wonderful
invention and a most striking demonstration. This was Otto von Guericke
(1602-1686), Burgomaster of Magdeburg, and councillor to his "most
serene and potent Highness" the elector of that place. When not
engrossed with the duties of public office, he devoted his time to the
study of the sciences, particularly pneumatics and electricity,
both then in their infancy. The discoveries of Galileo, Pascal, and
Torricelli incited him to solve the problem of the creation of a
vacuum--a desideratum since before the days of Aristotle. His first
experiments were with a wooden pump and a barrel of water, but he soon
found that with such porous material as wood a vacuum could not be
created or maintained. He therefore made use of a globe of copper, with
pump and stop-cock; and with this he was able to pump out air almost as
easily as water. Thus, in 1650, the air-pump was invented. Continuing
his experiments upon vacuums and atmospheric pressure with his newly
discovered pump, he made some startling discoveries as to the enormous
pressure exerted by the air.

It was not his intention, however, to demonstrate his newly acquired
knowledge by words or theories alone, nor by mere laboratory
experiments; but he chose instead an open field, to which were invited
Emperor Ferdinand III., and all the princes of the Diet at Ratisbon.
When they were assembled he produced two hollow brass hemispheres
about two feet in diameter, and placing their exactly fitting surfaces
together, proceeded to pump out the air from their hollow interior,
thus causing them to stick together firmly in a most remarkable way,
apparently without