68 



Station elevation. — The elevation above sea level adopted for a sta- 

 tion as the basis to which all pressure observations at the station are 

 correlated. 



Ohserved reading. — The direct result of the reading of an instru- 

 ment, uncorrected for any errors. 



Actual fressure. — Meaning the actual pressure of the air at a 

 barometer, as obtained from the observed reading after applying the 

 necessary corrections for temperature, gravity, and instrumental 

 errors. 



Station pressure. — ^A pressure corresponding to an " adopted or sta- 

 tion elevation " which may differ slightly from the actual elevation of 

 the barometer. When the " actual " and " station " elevations are the 

 same at any particular station, the removal correction will be zero and 

 the actual pressure and the station pressure are then numerically equal. 



Reduced pressure. — The actual or station pressure reduced to sea 

 level, or to some other specified plane. 



Correction for scale errors., capillarity., etc. — A mean difference 

 between the readings of a given instrument and those of the stand- 

 ard barometer duly corrected. This quantity embraces all outstand- 

 ing errors in the subdivision of the scale, or its total length; errors 

 in the adjustment of the sighting edge to the zero line of the vernier ; 

 errors of capillarity, imperfect vacuum, etc. 



Correction for temperature. — The correction depending on the 

 temperature of the mercury and the metallic scale. 



Correction for local gravity: 



(a) Latitude term. — The correction based on the variation of 



the force of gravity with latitude. 

 (&) Altitude term. — The correction based on the variation of 

 gravity with altitude above sea level. 



Removal correction. — The correction necessitated by the removal 

 of an office, and based on the difference between the actual elevation 

 of the barometers in the new location and the adopted elevation for 

 the station in question. Strictly speaking, the " removal correc- 

 tion " varies from day to day, or from winter to summer, with large 

 changes of temperature, but this variation, in most cases, can be 

 neglected. When, however, the change in elevation is large, and 

 especially when great differences of temperature, as between winter 

 and summer, are considered, the change in the value of the " removal 

 correction " may exceed 0.01 inch or more, and it then becomes 

 desirable to employ a " variable " correction, as more fully explained 

 in paragraph 182 {a). 



Sum of corrections. — A term embracing all the corrections that 

 are practically constant for a given instrument in a given location, 

 namely, the correction for scale errors, capillarity, and gravity, and 

 the removal correction. This sum is given on the certificate of cor- 

 rections (Form No. 1059-Metl.) furnished for each instrument. 



Total correction. — The correction for temperature, plus the " sum 

 of corrections '" as defined above. 



Reduction to sea level. — The quantity which must be added to the 

 " actual " or " station " pressure in order to obtain the " reduced " 

 pressure. 



Reduction for elevation. — A quantity which must be added to or 

 substracted from the pressure at a given elevation in order to deduce 

 therefrom the pressure appropriate to some other specified elevation. 



