airhead

The only difference between an airhead and a beachhead is that an airhead covers 360 degrees whereas a beachhead usually covers 180 degrees. An airhead is two beachheads back to back with the reinforcements and resupply coming by air instead of by sea. … [T]o handle their mountainous proportions there must be in the airhead a sufficient number of trained and equipped airhead service troops.

[T]he mine then becomes exposed to the most fearful results, where the workings have been opened, by the Air being driven backwards along the Airhead into the reservoirs of Gas formed in the upper cavities of the workings, and issuing into the Gate-road charged with the Gas to the firing point, causing an explosion, of which many familiar instances might be adduced.