PRESSUE.E 49 



6121. To avoid errors of parallax when the aneroid barometer is read, 

 the line of sight should be perpendicular to the index hand on the instrument. 

 If the aneroid has a mirror surface on the dial, parallax may be avoided by 

 viewing it in a position such that the index hand and its image coincide. 



6200. BAROGRAPHS 



6210. GENERAL. The barograph is a continuous recorder of atmospheric 

 pressure. A pen moves across the chart 2.5 inches of distance for each 

 inch of mercury (1 inch equals 33.864 millibars of pressure). The barograph 

 consists of an aneroid pressure unit, suitable linkage and a clock driven 

 drum upon which the chart is fastened, A knurled knot at the top of the 

 bellows housing provides a means of setting the pen to the proper pressure 

 (see Figures 6 and 7). 



6211. Barographs furnished or tested by the Weather Bureau are ad- 

 justed to read sea-level pressure when installed aboard ship. The barograph 

 should be adjusted to read sea -level pressure each time the instrument 

 chart is changed. 



6212. For purposes of the weather observation, the atmospheric pres- 

 sure, as entered on Form 615-5 will not be obtained from the barograph, but 

 will be read from the barometer. The 3-hour pressure tendency, however, 

 will be read from the barograph trace (see J6300). 



6220. READING THE BAROGRAPH. The distance between horizontal 

 pressure lines on the chart is equivalent to 1.0 millibar of pressure. For 

 convenience of reading, each five millibar line is identified by bold type 

 numerals. Barometric pressure will be read to the nearest 0.1 millibar of 

 pressure. Three-hour pressure tendencies should be determined in accord- 

 ance with the instructions contained in II6300. 



6221. When the pressure changes an amount sufficient to cause the pen 

 arm to reach the top or bottom of the chart, turn the thumbnut on top of the 

 aneroid cell enough to move the pen approximately 30 millibars toward the 

 center of the chart. Renumber the pressure lines accordingly. After the 

 pressure has returned to normal, move the pen approximately 30 millibars 

 in the opposite direction. Adjust the pen, making sure that it checks with the 

 corrected sea-level pressure reading of the ship's aneroid barometer. 



6230. BAROGRAPH CHARTS - FORM WB 455-12. The Weather 

 Bureau will supply barograph charts upon request. All requests for charts 

 should include the Weather Bureau form number. If charts do not have a 

 Weather Bureau form number, the Weather Bureau should be consulted be- 

 fore the forms are ordered. Requests for forms should be made in accord- 

 ance with J1301. 



6230.1. The barograph chart should always be changed immediately 

 following the 0000 GMT observation of the first of each month, and each 

 time the clip bar on the drum approaches within 3 to 6 hours of the pen. 



