[Illustration: FIG. 157.—Details of bevel gear and arrow.]
The Dial—This is made of tinned iron sheet or of 1/4-inch wood nailed to 1/2-inch battens. It is held up to the post by 3-inch screws passing through front and battens. At the points of contact, the pole is slightly flattened to give a good bearing; and, to prevent the dial being twisted off by the wind, strip iron or stout galvanized wire stays run from one end of a batten to the other behind the post, to which they are secured.
The post should be well painted, the top protected by a zinc disc laid under the top bracket, and the bottom, up to a point 6 inches above the ground level, protected by charring or by a coat of boiled tar, before the dial and the brackets for the vane rod to turn in are fastened on. A white dial and black arrow and letters will be most satisfactory against a dark background; and vice versa for a light background. The letters are of relatively little importance, as the position of the arrow will be sufficient indication.
It gives little trouble to affix to the top of the pole 4 arms, each carrying the initial of one of the cardinal points of the compass. The position of these relatively to the direction in which the dial will face must be carefully thought out before setting the position in the ground. In any case the help of a compass will be needed to decide which is the north.
Having set in the post and rammed the earth tightly round it, loosen the bracket supporting the vane rod so that the vane bevel clears the dial bevel. Turn the vane to true north, set the dial arrow also to north, and raise the bevel so that it meshes, and make the bracket tight.
Note.—In the vicinity of London true north is 15 degrees east of the magnetic north.
The pole must be long enough to raise the vane clear of any objects which might act as screens, and its length will therefore depend on its position. As for the height of the dial above the ground, this must be left to individual preference or to circumstances. If conditions allow, it should be near enough to the ground to be examined easily with a lamp at night, as one of the chief advantages of the system is that the reading is independent of the visibility of the vane.
A Dial Indoors.—If some prominent part of the house, such as a chimney stack, be used to support the pole—which in such a case can be quite short—it is an easy matter to connect the vane with a dial indoors, provided that the rod can be run down an outside wall.
An Electrically Operated Dial.—Thanks to the electric current, it is possible to cause a wind vane, wherever it may be set, to work a dial situated anywhere indoors. A suggested method of effecting this is illustrated in Figs. 158 to 161, which are sufficiently explicit to enable the reader to fill in details for himself.
[Illustration: FIG. 158.—Plan and elevation of electric contact on vane post.]