Bats and the descent of bodies through the air
Bat wings with their 'rudders' a b; how a flat board descends and how a bird makes itself heavy or light.
This page pairs studies of bats with the general physics of bodies moving through air. Two figures at the upper right show a bat with its wings spread and with them folded, labelled a b for the 'rudders' of the bat's wings. The main column argues that a flat board of uniform weight descends uniformly when level and obliquely when tilted, and that the shape of a body's leading or trailing edge, together with any non-uniformity fore or aft, is what bends its straight path through air or water. A closing note explains how a gliding bird makes itself heavy or light at will by spreading or drawing in its wings and tail.
On this page
The 'rudders' of a bat's wings
Two figures show a bat with wings spread and with wings folded. The parts marked a b are identified as the rudders of the wings of bats.
How a flat board descends through the air
A parallel-sided board of uniform thickness and weight, laid flat in air of uniform resistance, descends uniformly in every part; tilted, it descends with uniform obliquity. The shape of the leading or trailing edge of a body moving through air or water is what bends its straight course, and any non-uniformity joined before or behind its extremes steers it right, left, up, or down.
Making itself heavy or light at will
A gliding bird makes itself heavy or light as it wishes: it spreads its wings and fans out its tail when it wants to slow a swift descent, or draws its wings in with its tail when it wants to speed a slowed descent.