The compressed air is contained in it.

At the present moment it is subjected to a pressure of eighty-seven pounds to the square inch.
If that hat should explode while I am sitting here, it would blow the roof off of this building." "So it killed you I wouldn't care." "Well, sir, the way I work this wonderful appliance is this: The air-pump is concealed in the small of my back, under my coat.

A pipe connects it with the receiver in my hat, and there is a kind of crank running down my right trouser leg and fastened to my boot, so that the mere act of walking pumps the air into the receiver.

But how do I effect the cooling process?
Listen: Another pipe comes from the receiver and empties into a kind of a sheet-iron undershirt, perforated with holes, which I wear beneath my outside shirt--" "If you'd wear something _over_ that shirt, so as to hide the dirt, you'd be more agreeable." "Now, s'posin' it's a warm day.

I'm going along the street with the air-crank in operation.