Another, slightly more technical, look at Cayley’s Four Forces of Flight!
Cayley’s Four Forces
We’ve all heard that a shark must swim continuously or sink. Do you know why? It’s because sharks do not have a swim bladder. “This gas-filled sac provides buoyancy and helps to keep the fish afloat by keeping it in a neutrally buoyant state.” When in this state, the fish is neither rising nor sinking in the water.
Many bony fish – those having skeletons of bone rather than cartilage like our shark friends – have a swim bladder. It is located in the dorsal portion of the fish as noted in the anatomical sketch included in this post. The swim bladder can expand or contract by filling with or emptying itself of gas through the use of the gas gland. (In some less developed types of fish, the fish fills or empties the swim bladder with gulps of air.) The gas never passes through the wall of the swim bladder. You can think of the swim bladder as a sort of internal helium balloon.
Which of these is the less viscous fluid?
In 1644, Evangelista Toricelli wrote, “We live submerged at the bottom of an ocean of air.” We don’t feel the force of the pressure of this fluid any more than aquatic creatures feel the force of the water on all sides. Why? Because there is a uniformity of pressure in both cases; gravity exerts pressure on all sides.
Imagine for a moment that everything on the Earth and above its surface could exist under the water, or vice versa, without any change in appearance or properties. If we visualize horizontal bands atop one another, it would break the habitable area into observable layers. Grass, trees, plants, insects, ground-dwelling animals would all be in the same layer as the plants, crabs, bottom-feeders, and sand dwelling creatures beneath the surface of the ocean. There would be fish swimming in the layer above our heads with the birds. Airplanes would soar further above, in a layer with the whales. Dolphins would escape the surface of the ocean, accompanied by rockets, at the topmost layer above Earth. It would be a jumbled and magnificent scene. Wait! There’s more!
Remember Sam? The Rhesus monkey who took a spin on Little Joe 2 as part of the Mercury program back in the early 60’s? Here he is, before and after the flight, showing off his model of the Mercury fiberglass contour couch. Wait! There’s more!