How did the 1747 goatskin-bag submarine design anticipate modern ballast tanks?

The 1747 goatskin-bag design anticipated modern ballast tanks by introducing a mechanical method to manipulate the vessel's buoyancy through the controlled intake and expulsion of water.1

In this early design, goatskin bags were attached to the hull and connected to apertures at the bottom of the craft. To submerge, the inventor filled the skins with water, increasing the vessel's weight and overall density to overcome its buoyancy. To return to the surface, a "twisting rod" was used to force the water out of the skins, effectively reducing the vessel's weight.1

This process mirrors the fundamental operation of modern submarines, which submerge by flooding main-ballast tanks with seawater to increase their density. To surface, modern vessels use compressed air to blow the water out of these tanks, restoring positive buoyancy. Both systems rely on the same physical principle: adjusting the craft's weight relative to the water it displaces to control its vertical movement.23