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.]

In-this case the vane is attached (Fig. 158) to a brass tube, closed at the upper end, and supported by a long spike stuck into the top of the pole.  A little platform carries a brass ring, divided into as many insulated segments as the points which the vane is to be able to register.  Thus, there will be eight segments if the half-points as well as the cardinal points are to be shown on the dial.  The centre of each of these segments lies on a line running through the centre of the spike to the compass point to which the segment belongs.  The tube moves with it a rotating contact piece, which rubs against the tops of the segments.

Below it is a “brush” of strip brass pressing against the tube.  This brush is connected with a wire running to one terminal of a battery near the dial.

[Illustration:  Fig. 159.—­Magnetic recording dial.]

The Dial.—­This may be either vertical or horizontal, provided that the arrow is well balanced.  The arrow, which should be of some light non-magnetic material, such as cardboard or wood, carries on its lower side, near the point, a piece of soft iron.  Under the path of this piece is a ring of equally spaced magnets, their number equaling that of, the segments on the vane.  Between arrow and magnets is the dial on which the points are marked (Fig. 159).