47 



CHAPTER VI. PRESSURE 



6000. GENERAL 



6001. Atmospheric pressure is the pressure exerted by the weight of a 

 column of air, of unit area, extending vertically from a given surface (sea 

 level, for instance) to the top of the atmosphere. Atmospheric pressure may 

 be measured with aneroid or mercurial barometers and aneroid barographs. 



6010. INSTALLATION OF BAROMETERS. Barometers used in taking 

 weather observations should be mounted in the chart room. The location 

 should be as free as possible of vibrations and rapid changes in temperature. 

 It should not be mounted close to sources of artificial heat, such as steam 

 pipes or electric lights. The barometer should be placed at about the 

 eye level of the observer. 



6020. BAROMETER CORRECTIONS, To insure comparability of pres- 

 sure data, the Weather Bureau tests and assigns a number to each barometer. 

 The correction applicable to the barometer is entered on a tag, WB Form 

 455-2. The correction tag should be posted in a convenient place near 

 the barometer and the correction applied to each barometer reading. The 

 amount of the correction should be entered opposite the caption "correction" 

 at the head of column 13 of each Form 615-5. A revised correction should 

 be obtained at least once every 3 months. The correction is determined 

 by comparing readings of the ship's barometer with pressure values obtained 

 from a standard barometer. For this purpose, a precision aneroid may 

 be carried aboard ship by a Weather Bureau representative, or the com- 

 parison may be made by telephone contact with one of the port offices listed 

 in 3J1400. When comparisons indicate that a barometer is defective, the 

 Weather Bureau representative will recommend that it be replaced or 

 repaired. 



6021. When barometric comparisons cannot be made directly or by 

 telephone. Form 455-1 "Request for Barometer Comparison" may be filled 

 out in accordance with the instructions printed on it and mailed to the near- 

 est Port Meteorological Office. Upon receipt of the request, a Weather 

 Bureau representative will visit the ship during the ship's inport period 

 and compare all ship's barometers. A barometer correction tag will be 

 attached to each barometer corrected. 



6100. ANEROID BAROMETERS 



6110. GENERAL. Pressure is indicated on an aneroid barometer by 

 the position of a hand on a graduated dial. Aneroids have dials graduated at 

 intervals equivalent to inches and hundredths of mercury, millibars and 



