For a number of purposes it is convenient to have a platform floating at a constant level high up in the atmosphere. This can be contrived by the use of a special arrangement of balloons for heights up to about 100,000 ft. There are at least three ways in which this can be achieved. In the first an extensible balloon is filled with sufficient gas to give it buoyancy for a definite rate of ascent. As it ascends and the atmospheric density decreases, the gas and the balloon expand and if the ascent goes on long enough the balloon bursts. If when the balloon reaches a certain dimension (or alternatively when the exterior pressure falls to a certain figure), a valve opens and allows sufficient gas to escape to stop the ascent, the balloon will float in equilibrium. A second method is to use a non-extensible balloon only partially filled with gas and with an open mouth; as the balloon ascends, the gas expands to fill the whole balloon and overspills thereby reducing the rate of ascent until equilibrium is reached. A third method involves the use of a number of extensible balloons tied together. When they have risen in the air and the balloons have stretched, some will burst before others and thereby a rough equilibrium is attained and the remaining balloons will float at nearly a constant level. A variant of this is to detach some of the balloons from the assembly at a predetermined pressure level, so that the remainder are just able to support the instruments and equipment.