PSYCHROMETER . An instrument used for measuring the 

 water vapor content of the atmosphere; a type of 

 hygrometer. It consists of two thermometers, one 

 of which (dry bulb) is an ordinary glass thermome- 

 ter, while the other (wet bulb) has its bulb 

 covered with a jacket of clean muslin which is 

 saturated with distilled water prior to an observa- 

 tion. When the bulbs are suitably ventilated, 

 they indicate the thermodynamic WET- and DRY-BULB 

 TE^fPERATURE of the atmosphere. (14) 



FT. U. S. 

 Boat. 



Navy designation for a Motor Torpedo 



PW. Pulse Width, 



PY. U. S. Navy ship designation for a Yacht. 



PYRAMIDAL ICEBERG . See PINNACLED ICEBERG. 



PYROCLASTIC . Solid material ejected from a volcano, 

 ranging from large volcanic bombs to fine volcanic 

 ash and dust. (27) 



PTC . U. S. Navy ship designation for Motor Sub 

 Chaser. Also Pacific Tuna Conference. 



PTEROFODS . Pelagic, swimming-type gastropods in 

 which the foot is modified into a pair of winglike 

 lobes or fins. Pteropod ooze contains conspicuous 

 shells of these pelagic mollusks and is an impor- 

 tant constituent of the deep ocean deposits. 

 Pteropod ooze usually is classed as mud on bottom 

 sediment charts. (16) 



FTP . 

 Boat, 



U. S. Navy designation for a Fast Patrol 



PULSE RISE TIME . The pulse rise time is the inter- 

 val of time required for the leading edge of a 

 pulse to rise from some specified small fraction to 

 some specified larger fraction of the maximum value. 

 (2) 



PUMICE . An excessively cellular, glassy lava. It 

 is very light and floats on water until it becomes 

 water logged and sinks. Pumice is classed as 

 gravel on bottom sediment charts. (16) 



PYRHELIOMETER . General term for the class of 

 actinometers which measure the intensity of direct 

 solar radiation. 



The instrument consists of a radiation sensing 

 element enclosed in a casing which is closed except 

 for a small aperture, through which the direct 

 solar rays enter, and a recorder unit. Pyrhelio- 

 meters can be classified on the basis of the sens- 

 ing elements employed. In one form the sensing 

 element is a blackened water calorimeter. The rise 

 in the temperature of the water gives a measure of 

 the amount of radiant energy absorbed during the 

 exposure of the instrument. Another type of sens- 

 ing element consists of a blackened plate of high 

 heat capacity. When radiation is allowed to fall 

 on the plate for a period short compared to the 

 thermal time constant, the temperature rise of the 

 plate is proportional to the intensity of the in- 

 coming radiation. A third type of sensing element 

 consists of a pair of plates, one blackened and one 

 reflecting, which are continuously exposed to the 

 incoming radiation. The temperature differential 

 between the plates is proportional to the intensity 

 of the incoming radiation. (24) 



Q (QUALITY FACTOR) . The quantity Q is a measure of 

 the sharpness of resonance or frequency selectivity 

 of a resonant vibratory system having a single 

 degree of freedom, either mechanical or electrical. 

 A high value for Q means that the resonance is 

 sharp; i. e., and small change in the frequency of 

 the excitation causes a large drop in the response 

 of the system. (9) 



QUARANTINE BUOY . A buoy marking the location of a 

 quarantine anchorage. In U. S. waters a quarantine 

 buoy is yellow. (17) 



QUARTERING SEA . Waves moving in a direction ap- 

 proximating 45° from a vessel's heading, striking 

 the vessel on the quarter. Those moving in the 

 general direction of the heading are called follow- 

 ing sea, those moving in a direction approximately 

 opposite to the heading are called head sea, and 

 those moving in a direction approximately 90° from 

 the heading are called beam sea. (17) 



source. When the potential of the upper face of 

 the crystal is positive, the thickness increases. 

 Simultaneously, the other two dimensions shrink. 

 Since the plate will be compressed during one half 

 of the cycle of an ac field, and extended the same 

 amount during the other half, it will vibrate with 

 the same period as that of the field. If this is 

 the natural period of the crystal, the amplitude 

 of the vibrations will be a maximum. (30) 



QUAY . A WHARF approximately parallel to the shore 

 line and accommodating ships on one side only, the 

 other side being attached to the shore. It is 

 usually of solid construction, as contrasted xjith 

 the open pile construction usuallv used for PIERS. 

 (17) 



QUENCHED WATER . There are many occasions when the 

 apparent propagation loss of some water path, of 

 only moderate length, will be such as to indicate 

 an attenuation much greater than would be predicted. 

 Such abnormally high attenuations are attributed to 

 what has come to be called quenched water. This is 

 a condition often encountered in shallow water, or 

 near shores where there are strong currents accom- 

 panied by considerable turbulence. It is believed 

 that the excessive reductions in acoustic intensity 

 are the result of occluded air. (4) 



QUARTZ PROJECTORS . X-cut quartz crystals are laid 

 flat on a steel plate, arranged in a mosaic so that 

 the plate is adequately covered. An identical 

 plate is then laid on top of the crystals, forming 

 a sandwich. Insulating washers make it possible 

 to connect the plates to the terminals of the ac 



QUICKSAND . Sand which has lost its grain-to-grain 

 contact by the buoyancy effect of water flowing 

 upward through the voids. Such material, having 

 some of the characteristics of a fluid, possesses 

 no load-bearing value. It is a condition and not 

 a type of material. (27) 



95 



