The static buoyancy of airships in flight is not constant. It is therefore necessary to control the altitude of an airship by controlling its buoyancy: buoyancy compensation.
On the Macon, the exhaust water recovery condensers appear as dark vertical strips above each engine. The Akron and LZ 130 Graf Zeppelin had similar systems.
LZ 130 Graf Zeppelin
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The Graf Zeppelin was the last of the German rigid airships built by Zeppelin Luftschiffbau during the period between the World Wars, the second and final ship of the Hindenburg class, and the second zeppelin to carry the name "Graf Zeppelin" and thus often referred to as Graf Zeppelin II. Due to the United States refusal to export helium to Germany, the Graf Zeppelin II was inflated with hydrogen and therefore never carried commercial passengers. It made 30 flights over 11 months in 1938–39, many being propaganda publicity flights; but staff of the Reich Air Ministry were aboard to conduct radio surveillance and measurements. The airship, along with its LZ 127 namesake were both scrapped in April 1940, and their duralumin framework salvaged to build aircraft for the Luftwaffe.
LZ 130 under construction with tractor-type engine cars installed
''Graf Zeppelin'' in flight
Postcard carried from Frankfurt (Rhein-Main) to Reichenberg (Sudetengau) on the "Sudetendeutsche Freiheitsfahrt 1938" on the first mail flight of the "Graf Zeppelin II" (DLZ130), December 1–2, 1938