This is the only table hitherto much in use for converting velocity into pressure, and was prepared by Smeaton and others. It does not, however, express the true relation, which has yet to be determined.
The Anemograph, or Self-Recording Wind Gauge, has for its object the registration of the velocity and direction of the wind from day to day. Figs. 59 and 60 show the form designed and arranged by Mr. Beckley, of the Kew Observatory, which has been adopted by the Meteorological Office.
[Illustration: 59. Anemograph. Scale about 1/20. Portion for exterior of observatory.]
It consists of a set of hemispherical cups and vanes, which are exposed on the roof of the house, and of the recording apparatus, which is placed inside the house.
The motion imparted to the hemispherical cups by the wind is communicated to the steel shaft B, which, passing through the hollow shaft C, and having at its lower end an endless screw, works into a series of wheels in the iron box D, which reduces the angular velocity 7,000 times. At the required distance the motion, having emerged at E, is connected with F, where, by means of bevelled wheels, it moves the spiral brass registering pencil C, which is arranged so that each revolution records 50 miles of velocity on the prepared paper H.
The direction of the wind is indicated by the arrow L, which is kept in position by the fans M. These communicate, by an endless screw and train of wheels, through the shaft C and the box D to the recording apparatus, consisting of a spiral brass pencil, which in one revolution records variations through the cardinal points of the compass, on the same prepared paper as that which receives the record of velocity.
[Illustration: 60. Anemograph. Scale about 1/20. Portion for interior of observatory.]
The paper is held on the drum by two small clips, and may be readily changed, by unclamping the cross V, without disturbing the drum or any other part of the instrument.
[Illustration: 61. Self-recording Magnetometer, Kew Observatory.]
William Gilbert, a physician of Colchester, first showed in 1600 that the earth as a whole has the properties of a magnet, and consequently that the directive action exerted by it upon a compass needle represents only a special case of the mutual action of two magnets. In 1845, Faraday established the fact that susceptibility to magnetic force is not, as was generally believed, confined to iron, nickel, and a few other substances, but is a property of all substances. According to Balfour Stewart, auroræ and earth currents may be regarded as secondary currents resulting from changes in the earth’s magnetism. Magnetic phenomena are included under the general term terrestrial magnetic elements, and consist of magnetic declination, inclination, and intensity.