280. =Balloons.=--The first balloons that were used were filled with heated air. You have already seen, in § 149, why it is that balloons rise. Now in the hot-air balloon it is the expansion of the air by heat that makes it lighter than the surrounding air. Of course such a balloon is not as effective as the gas balloon, for the air within it loses its comparative lightness as it becomes cooled; while the gas which is used, being very much lighter than air at the same temperature, does not lose its lightness as the balloon goes up. You learned in § 152 that the atmosphere becomes thinner as we go upward. The gas balloon, therefore, rises until it arrives at that point where the air is of about the same specific gravity with the gas, and there it stops. It is made to descend by letting out some of the gas from a valve. Gas was not used for balloons till 1782. Hydrogen gas was employed at first, being over fourteen times lighter than air. Of late the common burning gas, carbureted hydrogen, has been generally used, because it can be so readily obtained where there are gas-works. [Illustration: Fig. 197.] 281. =Currents in the Air from Heat.=--Heat is the grand mover of the atmosphere. Any portion of it that becomes warmer than surrounding portions rises, or rather is pushed up, for the same reason that a hot-air balloon rises, the only difference between the two cases being that in the one the air is confined, and in the other is left free, and so becomes diffused. And it is this rising of the air from expansion that causes nearly all the movements that we witness in the air. We see this exemplified in various ways wherever there is a fire. The air that is heated by the fire is forced upward by the colder air, which, on the principles of specific gravity, seeks to get below the warmer and lighter air. The hot air that comes through the registers of a furnace is pushed up by colder air below. For the same reason the heated air around a stove-pipe is constantly going upward. This is very prettily shown by the toy represented in Fig. 197 (p. 218), which is a paper cut spirally, and suspended, as you see, upon the point of a wire. The upward current makes the paper revolve rapidly around the wire. It is from the rising of warm air that the galleries of a church are warmer than the space below. In a common room the disposition of the air is continually to have its warmest portions above and the colder below. It is for this reason that we have our arrangements for producing or introducing heat at as low a point as possible.