the direction of each being noted. If the barometer is steady and the sky is free from clouds, the direction shown by a wind vane may be taken as the direction of the wind to the height of ordinary flight altitudes. Neither the surface winds nor the cloud winds indicate definitely the presence of the updraughts and downdraughts which constitute bumps and air holes.
Robinson anemometer, electrically connected with recording apparatus, P. 239.
The Wind Vane.—The wind vane of the spread-tail pattern, used by the Weather Bureau, is probably the most practical form in use. It holds steadily to the wind; it is sensitive enough to respond to a breeze of 2 miles an hour. The regulation vane is 6 feet over all. The tail is made of very thin board strips, thoroughly weather-proofed; the metal work is rust-proof; the bearings are constructed so that friction is reduced to the minimum. This is the general service vane designed to show
