152 NIOHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE

It consists of an open mouthed canister 5 inches in diameter as in the
figure. The raindrops fall into the can and pass through the funnel
into the jug. The rain water is poured from the jug into the measuring

Fig 18.—Rain Gauge.
glass, which is scaled off into hundredths of an inch of rainfall. One
inch of rain caught in the rain gauge represents a rainfall of 101 tons
per acre, or its equivalent volume of 3630 cubic feet of water.
RADIO DIRECTION FINDER
Directional wireless as an aid to navigation is a usual part of the
equipment of a well found ship. Figure 19 illustrates the apparatus
supplied by the Radio Communication Company, London. The frame,
or loop aerial on the bridge is rotated by means of the handwheel
fitted to the lower end of the vertical spindle, which extends down from
the aerial through the deck to the chartroom below. The frame aerial
when rotated picks up the wireless wave, the signal strength reaching
its loudest when the plane of the frame aerial is parallel to the incoming
signal, that is, when it is edge-on to the direction of the station from
whence it is being transmitted.
The position of maximum strength is not sharply defined, but on
rotating the frame the signal strength gradually drops to inaudibility
when the frame is facing the direction of the station. A well
defined "silence arc'* of a few degrees on each side of the required
direction is established and by rotating the frame either way the signal
ugain rises to audibility. It is then a simple matter to observe by the