Plume. The feather-shaped echo produced on a radar display by an Echo Box.
Pluviograph. Self-recording rain gauge.
Ply. To work to windward. 2. To sail regularly between two ports. 3. A strand in cord or rope.
Pneumercator Gauge. Instrument for indicating depth of a liquid, or draught of a ship, by denoting the pressure necessary to balance the gravitational pressure of the liquid - the pressure being indicated by a gauge or a balancing column of mercury. This pressure is read off against a specially-calibrated scale.