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.
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