22 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



iiearcst Woatlier Bureau ofHt^-c lo prevent, so far as possible, the 

 issuance of untrustworthy barometers to ships of their lines. In 

 many instances barometers ai-e now purchased subject to replace- 

 ment by dealers if found unreliable. The tendency, therefore, is 

 in favor of a stricter attitude toward the replacement of old and 

 erratic or usage-worn barometers by new instruments on shipboard, 

 and toward an insistence that new barometers be proven satisfactory 

 before acceptance for installation. 



This action has resulted in a decided betterment in marine baro- 

 metric equipment and in an equally' positive improvement in the 

 accuracy of marine meteorological observations. 



BAROMETER COMPARISONS 



Notwithstanding that the greatest care may be taken in the con- 

 struction and handling of barometers certain errors due to various 

 causes can hardly be eliminated. In the case of mercurial barometers 

 the errors are usually small and fairly constant. In aneroids, how- 

 ever, the instrumental error may be (juite large and undergo irregular 

 changes. 



In order to obtain satisfactory readings of barometers it is neces- 

 sary to find the error of each instrument and apply a correction of 

 the opposite sign. The errors are determined by comparison with a 

 standard instrmnent. In order to obtain such a comparison it is only 

 necessary to make and record several readings of the ship's barome- 

 ter when in some port where a standard instrument is located, the 

 i-eadings being made at the same liours at which the latter is read. 

 In United States ports the barometer is read each day at 7:30 a. m. 

 and 7 : 30 p. m,, 75th meridian 1 ime. In European ports observations 

 are made generally at 7 and 18 hours, Greenwich mean time. Such 

 observations ultimately reach the Weather Bureau and are available 

 for use in checking comparative readings. 



In making comparisons it is desirable that at least three readings 

 of the ship's barometer should be made at uniform intervals of 12 

 or 24 hours. During the comparison the instrument should hang 

 in its accustomed ]^lace aboard ship. If possible the readings should 

 be made by the officer charged with the duty of making the meteoro- 

 logical observations. 



Blank cards, known as barometer comparison cards (Form No. 

 1202-Marine), for recording com])arative readings are supplied by 

 the Weather Bureau. A sample card, filled out and with corrections 

 made, is shown on page 24. 



The successive readings of the l)arometer, at the hours selected by 

 the observer, are entered in column 1 of the barometer comparison 

 card. The readings of the attached thermometer will be entered in 

 the adjacent column. The card should then be dispatched to the 

 nearest office of the Weather Bureau. 



At the Weather Bureau office the readings of mercurial barom- 

 eters are corrected for temperature and reduced to standard gravity 

 (latitude 45°), the results being entered in column 2. In column 3 

 are given the corresponding readings of the standard Weatlier Bu- 

 reau barometer, likewise corrected for temperature and reduced to 

 s-tandard gravity, as well as to sea level. 



