The air has a density, anything less dense than it can float there-just as a piece of wood, less dense than water, floats. Some gases, such as helium and hydrogen, are less dense than air. A balloon or airship inflated to one of these gases floats in the air. Hydrogen is the least dense of all gases but, extremely flammable, it is also of a dangerous use. Probe balloons and airships are usually inflated with helium, non-flammable. Heated, a gas expands and loses its density. it is therefore possible to build a balloon and inflate it with hot air. Balloons are used for sporting and recreational purposes.
Hot Air Balloon
The first hot-air balloons were designed in 1783 by two Frenchmen, the Montgolfier brothers, who were papermakers, so that the first balloons were made of paper or silk. Nowadays, the balloon is made of synthetic fabrics.
A butane burner, at its base, sends hot gases into the balloon and the passengers settle in a platform suspended under the burner. Modern balloons have reached altitudes above 35 km.
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