INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 9 



PART III. INSTRUMENTS AND INSTRUMENTAL 

 OBSERVATIONS 



Few ships carry meteorological instruments other than barometers 

 and thermometers. Readings of the barometer, showing the atmos- 

 pheric pressure and its changes, are of the highest importance. 

 Readings of thermometers, showing the air and water temperatures, 

 and also the depression of the "wet bulb", from which the humidity 

 is determined, are of great value. 



The following instructions relate to the care, exposure, and read- 

 ing of barometers and thermometers and their comparison with 

 standard instruments. Additional information regarding the proper 

 methods of making weather observations on shipboard with instru- 

 ments will be found in part IV, where the preparation of the com- 

 plete weather report, including noninstrumental observations, is 

 explained in detail. 



MERCURIAL BAROMETERS 



Mercurial haroineter of manne type. — The difficulties encountered 

 in the use of the ordinary type of mercurial barometer on ship- 

 board, owing to motions of the ship, have been overcome in a form 

 of instrument known as the Kew, or marine, barometer. Its dis- 

 tinguishing characteristics consist in substituting for the simple 

 straight tube of uniform bore commonly employed in land barome- 

 ters, a tube having a wide bore for 6 or 8 inches of the upper pbr- 

 tion only. Below this the tube has thick walls with a small capil- 

 lary bore only a few hundredths inch in diameter. Near the bottom 

 end the bore of the tube is again enlarged to form an air trap, all 

 as shown in figure 1. If small quantities of air chance to enter the 

 open end of the tube they are not likely to enter the small point of 

 the inner tube, but lodge instead in the surrounding space, as indi- 

 cated, where the air must remain and does not affect the barometric 

 readings. It may even be removed from the trap when the barometer 

 is undergoing repairs. 



The flow of mercury through the capillary bore takes place so 

 slowly that the column cannot surge up and down the tube seriously 

 with the relatively quick motions of the ship. At the same time 

 the height of the column adjusts itself to the slow changes of atmos- 

 pheric pressure, and thus more or less perfectly answers the desired 

 objects. 



Figure 2 shows a high-grade mercurial barometer adapted to all 

 the requirements of marine use, together with a special gimbal sup- 

 porting bracket and wooden box, into which the barometer and 

 bracket are folded and thoroughly protected and secured when not 

 in use. 



The glass tube and boxwood cistern, all as shown in figure 2, are 

 secured inside the bronze-metal jacket provided at the top with a 

 long, slotted opening through which the top of the glass tube and 

 mercurial column can be seen. A scale of graduations is fixed beside 

 \h.^ opening and a vernier is arranged to slide up and down so as to 

 enable accurate measurements of the height of the mercurial column 

 to be made. 



