The Siebe Closed Diving Helmet

The helmet is a pressure vessel you wear, and its shape is chosen for strength.

1. Cut a copper disc and raise it into a dome by hammering over a stake, working from the centre outward in concentric courses.
2. Anneal frequently — copper work-hardens fast and will split if you push it cold.
3. Planish over a smooth stake to even the surface and stiffen it.
4. Form a second dome and join them into a helmet with a neck opening and viewport apertures.

A dome carries external pressure in pure compression with no bending anywhere, which is why it is far stronger than a flat plate of the same thickness. Press on a flat sheet and then on a domed one and the difference is immediate.

Air must leave as fast as it arrives, and the diver controls buoyancy with that balance.

1. Fit an adjustable spring-loaded exhaust valve.
2. Feed air steadily and adjust the spring until internal pressure sits just above the surrounding water pressure.
3. Tighten the exhaust and watch the suit inflate; loosen it and watch it deflate.

The diver is adjusting his own displacement. Combined with weighted boots and a lead belt, that gives fine control of whether he rises, sinks or stands.

It also introduces blow-up. If a diver becomes too buoyant and starts rising, surrounding pressure falls, so the trapped air expands, so he becomes MORE buoyant. A runaway ending in an uncontrolled ascent — which is how divers get bent or embolised, and exactly the subject of the next blueprint.