72 



194. Comparison of private harometers. — Observers will extend 

 every courtesy to shipmaster's and others who may apply at the office 

 for information or submit barometers for comparison, adjustment, 

 etc. The mercurial barometers at a station can not as a rule be re- 

 moved from the office for comparison and adjustment of barometers 

 on board vessels, etc., but in some cases aneroid barometers serve very 

 conveniently for this purpose and may be supplied when r'equired. 



A memorandum or tag, showing date of comparison, the correction 

 to be applied, and other pertinent facts should accompany an instru- 

 ment that has been compared. 



VIII. Explanation of Tables 



TAHT.TilS I AND n. COERECTION FOK TEMPEIRATTJBE 



195. Tables I and II give, in English and metric measures, respec- 

 tively, the corrections that should be applied to the observed readings 

 of a mer'curial barometer having a brass scale, in order to eliminate 

 from the barometer reading the effect of temperature in expanding 

 the mercury and the scale. The following example will elucidate 

 the use of either table : 



The attached thermometer of a barometer reads 72. 5° 



The barometer reads 29. 415 



In Table I, the pressure nearest 29.415 is 29.5. Horizontally oppo- 

 site 72.5° in the vertical column under this pressure we find 0.117, 

 which is the correction required, and we note that it must be sub- 

 tracted. 



The reading corrected for temperature is, therefore 29. 41.5 



—.117 



29. 298 



196. Interpolation explained). — Suppose the pressure had been 

 29.281, which is about midway between 29 and 29.5. The correction 

 in this case should be about midway between 0.115 and 0.117, or 0.116. 

 The same rules must be followed if the temperature is intermediate 

 between the values of the table. 



TABLES III AND IV. INFLUENCE OF GBAVITY ON BAEOMETIC OBSEEVATIONS 



197. Table III gives the correction required to reduce readings of 

 the mercurial barometer to standard gravity at latitude 45°. 



There is, in addition to the above, another gravity correction 

 required to eliminate the effect of change of gravity with elevation. 

 This is small and is given in Table IV. 



The aneroid barometer does not require any gravity correction. 



Example: For latitude 38° and pressure 29 inches, the gravity 

 correction is ininus 0.018 of an inch. For an altitude of 2,490 feet 

 and a pressure of 27.3 inches the gravity correction is —0.004 of 

 an inch. 



TABLE V. — ^PRESSUEE IN INCHES CORRESPONDING TO CHANGES OF 100 FEET IN 



ELEVATION 



198. Table V is employed for computing " removal corrections," 

 paragraph 182, and its use is illustrated by the following example : 



Suppose the " adopted or station elevation " of a given station is 

 1,482.7 feet, and that after removal the "actual elevation" is 1,516.4 

 feet ; hence the change in elevation will be 33.7 feet. 



