than applied at a single point such as a seat or harness straps. This principle is used in a new type of G-suit called the Libelle G-suit, which allows aircraft pilots to remain conscious and functioning at more than 10g acceleration by surrounding them with water in a rigid suit. Acceleration protection by liquid immersion is limited by the differential density of body tissues and immersion fluid, limiting the utility of this method to about 15g to 20g. Extending acceleration protection beyond 20g requires filling the lungs with fluid of density similar to water. An astronaut totally immersed in liquid, with liquid inside all body cavities, will feel little effect from extreme G forces because the forces on a liquid are distributed equally, and in all directions simultaneously. However effects will be felt because of density differences between different body tissues, so an upper acceleration limit still exists. Liquid breathing for acceleration protection may never be practical because of the difficulty of finding a suitable breathing medium of similar density to water that is compatible with lung tissue. Perfluorocarbon fluids are twice as dense as water, hence unsuitable for this application. Examples in fiction Literary works Alexander Beliaev's 1928 science fiction novel Amphibian Man is based on a scientist and a maverick surgeon, who makes his son, Ichthyander (etymology: "fish" + "man") a life-saving transplant – a set of shark gills. There is a film based on the novel. L. Sprague de Camp's 1938 short story "The Merman" hinges on an experimental process to make lungs function as gills, thus allowing a human being to "breathe" under water. Hal Clement's 1973 novel Ocean on Top portrays a small underwater civilization living in a 'bubble' of oxygenated fluid denser than seawater. Joe Haldeman's 1975 novel The Forever War describes liquid immersion and breathing in great detail as a key technology to allow space travel and combat with acceleration up to 50 G. In the Star Trek: The Next Generation novel The Children of Hamlin (1988) the crew of the Enterprise-D encounter an alien race whose ships contain a breathable liquid environment. Peter Benchley's 1994 novel White Shark centers around a Nazi scientist's experimental attempts to create an amphibious human, whose lungs are surgically modified to breathe underwater, and trained to reflexively do so after being flooded with a fluorocarbon solution. Judith and Garfield Reeves-Stevens' 1994 Star Trek novel Federation explains that