INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS H 



For marine use it is ne'cessary that the barometer be free to hang 

 in a vertical line despite the rolling and pitching of the vessel. For 

 this purpose the well-known arrangement of gimbal rings is formed 

 upon the outer extremity of a hinged bracket and secured to the 

 barometer at a point some inches above the middle. 



In the position shown in the picture the barometer is ready for 

 reading, and the tube will swing on the gimbals so as to remain 

 nearly or quite vertical. After a reading has been taken the barom- 

 eter must not be left exposed, as it is very liable to injury by violent 

 oscillations in heavy weather. In the equipment of the standard 

 Weather Bureau design the whole bracket, barometer, and all are 

 arranged to fold up compactly within the small mahogany case, the 

 lid of which closes with a spring clasp, and not only secures the 

 barometer from accidental damage but from undue exposure to 

 atmospheric influences as well. 



Standard types of both land and marine mercurial barometers are 

 found in offices of the Weather Bureau at the principal United 

 States ports, and seamen are invited to visit such offices for the pur- 

 pose of familiarizing themselves with these and other meteorological 

 instruments. 



Explanation of scale of marine harometer.- — All marine barometers 

 are of the fixed-cistern type, as it is called, and the only setting 

 required is to bring the lower edge of the vernier accurately to the 

 level of the top of the mercurial column, whereupon the scale reading 

 giA^es directly the observed or uncorrected air pressure. This result 

 is realized by shortening the graduations on the scale so that instead 

 of representing true standard inches, millimeters, or millibars,^ as 

 the case may be, the graduations have such a value as to eliminate or 

 take account of the slight rise and fall of the level of the mercury in 

 I he cistern as the column rises or falls. 



If, for example, the column of mercury in the tube falls, say, 1 

 inch, there will be a rise of the mercury in the cistern, but the 

 amount will be small because the area of the cistern is so much 

 greater than that of the tube. For example, in the barometer from 

 which the illustration in figure 1 was prepared, the rise of the 

 mercury in the cistern for a fall of 1 inch in the tube amounts to 

 only about three-hundredths of an inch (0.03). Consequently, in 

 this barometer, an observed fall of 1 inch in the tube means a fall 

 of 1.03 inches in the pressure. If, now, we prepare a special scale 

 for this barometer so that each true inch of space on the scale 

 represents 1.03 inches of the arbitrary scale values, and, if we set 

 this scale so that the barometer reading at any one point of the scale 

 agrees exactly with the reading of a standard barometer alongside 

 of it, then the readings should agree closely at all other points of 

 the scale. Slight irregularities in the bore of the tube and diameter 

 of the cistern may introduce small errors ; otherwise, the contracted- 

 scale barometer, with settings made only at the top of the mercurial 

 column, is capable of yielding pressure readings of great accuracy, 

 and this artifice is universally employed in the ordinary marine 

 barometer. 



Errors of mercurial harometers. — No matter how carefully a 

 barometer may be made, certain errors due to various causes can 

 hardly be eliminated. In the first place, if any air or other gaseous 



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