IQ INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



As an example, let the observed reading of the mercurial barometer 

 on board a vessel in the Caribbean Sea, midway between Colon and 

 the Windward Passage, be 29.95 inches and the temperature as given 

 by the attached thermometer, 74° F. 



We then have — 



Inches 



Observed height of the mercury 29.95 



Correction for temperature, 74° (table 1) — 0.12 



29.83 

 Correction for gravity, latitude 15° (table 3) ^0.07 



29.76 

 Correction for height above sea level and instrumental error, assumed, 



(Barometer Tag) +0.06 



Corrected barometer reading 29. 82 



Figure 8. — ^Aneroid barometer. 



ANEROID BAROMETERS 



Figure 8 represents one of the more important types of aneroid or 

 holosteric barometers, showing, principally, the internal mechanism. 

 The essential feature consists of the small metallic box or cell, M, 

 the upper and lower walls of w^hich are made of very thin circular 

 sheets of corrugated German silver, which are soldered together on 

 their outer edges, forming a very short cylinder. The air is thor- 

 oughly exhausted from this cell through a tube at one side, which, 

 when the vacuum is as perfect as desired, is pinched tightly together, 

 cut off, and hermetically sealed with solder, producing the projection 

 seen at c. The flexible corrugated surfaces, which tend to be col- 

 lapsed by the pressure of the outside air, are forcibly held apart by 

 the action of a strong steel spring, R. As the pressure of the air 

 increases the spring is compressed and the corrugated surfaces ap- 



