INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 23 



The differences between the simuhaneous readings of the two 

 barometers are entered in cohimn 4, and the mean of these differences, 

 if accepted, is adopted as the instrumental error of the ship's barom- 

 eter. This is at once inscribed upon a barometer tag (Form No. 

 1203-Marine or 1203A-Marine), a Weather Bureau identification 

 number is given to the barometer, and the tag mailed to the observer. 

 A sample of the tag used for aneroid barometers, properly filled out, 

 is given below. A similar tag (Form No. 1203A-Marine) is used 

 for mercurial barometers. 



Form No. 1203— Marine 



U. S. DEPARTMENT OF AGRICULTURE 

 U. S. WEATHER BUREAU BAROMETER TAG 



VESSEL American S. S. QUIRIGUA 



(Nationality) (Kind) (Name) 



Aneroid Barometer No -^A.^.^ W. B. List 



COAIPARED 



Place New York D^^^ Apr, 12-13, 1957 



To determine sea-level (corrected) readings — 



Add Subtract— -:A^- 



(SEE REVERSE SIDE OF THIS TAG) 



Lack of agreement in the observations. — In case the range be- 

 tween the largest and the smallest individual corrections in the series 

 of comparative readings is more than 0.10 inch, the card is returned 

 to the observer with the statement that the several observations do 

 not agree sufficiently well among themselves to furnish a reliable 

 correction for the instrument, and with a request for another set of 

 readings. 



In some instances differences indicated by comparative readings 

 are due to errors on the part of observers, but more often are 

 occasioned by defects in the instrument. The true source of such 

 differences is generally revealed by a second set of comparative 

 readings, and if these indicate that the instrument is unreliable, it 

 should be repaired or replaced in the interest of safe navigation. 



Frequency of comparison. — On account of the severe usage a 

 barometer is subjected to aboard ship its instrumental error is likely 

 to change. Observers should, therefore, take advantage of every 

 opportunity to obtain a comparison with a standard instrument. 

 Aboard steam vessels such a comparison should be made at least 

 once in 3 or 4: months. If the j)erformance of the barometer ap- 

 pears to be questionable, more frequent comparisons are desirable. 



No attempt should be made to adjust a barometer. Such attempts 

 are likely to increase the irregularities of the instrument. It is 

 preferable to allow the errors to accumulate, the amount to be de- 

 termined by frequent comparison, as explained on page 19. 



