I.

Introduction

Balloon, lighter-than-air craft consisting of a large spherical, pliant bag made of varnished silk, rubber, or other suitable non-porous material, and containing either hot air or a gas that is lighter than air. Manned balloons carry one or more people in a suspended gondola; unmanned balloons carry instruments used to measure and record a variety of physical phenomena. The earliest balloons were filled with hot air and often carried a brazier to replenish the supply continuously. Modern balloons are filled with hydrogen or helium, or, in the case of modern hot-air sports ballooning, air heated by a small gas burner. Helium has the great advantage of being non-flammable, but it is twice as heavy as hydrogen and has 7 per cent less lifting power. Hydrogen weighs 1.14 kg/cubic m (0.071 lb/cu ft) less than air at standard atmospheric pressure and temperature, and a hydrogen balloon of 30 cubic m (1,060 cu ft) is able to lift, therefore, about 34 kg (75 lb). See Atmosphere.