10 



the formula above, and may also change the value of Rq, but these 

 effects are slight and are not considered here. 



26. Mercurial harotneteT for marine use. — Mercurial barometers 

 constructed upon the Fortin system and other forms such as described 

 in the preceding paragraphs are almost universally used on land for 

 the measurement of atmospheric pressure, and no other form of 

 barometer affords as great accuracy in the measurement of that pres- 

 sure. If, however, such an instrument were placed aboard ship at 

 sea the column of mercury would surge up and down the tube more 

 or less violently with every motion of the vessel, and readings would 

 be rendered inaccurate or impossible. 



27. This difficulty has been overcome in a form of instrument 

 known as the Kew, or marine, barometer. Its distinguishing char- 

 acteristics consist in substituting for the simple straight tube of 

 uniform bore commonly employed in land barometers, a tube having 

 a wide bore for 6 or 8 inches of the upper portion only. Below this 

 the tube has thick walls with a small capillary bore only a few hun- 

 dredths 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 11. 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 indicated, 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. 



28. The flow of mercury through the capillary bore takes place so 

 slowly that the column can not 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. 



29. Figure 12 shows a high-grade mercurial barometer adapted to 

 all the requirements of marine use, together with a special gimbal 

 supporting bracket and small wooden box, into which the barometer 

 and bracket are folded and thoroughly protected and secured when 

 not in use. 



30. The glass tube and boxwood cistern, all as shown in Figure 11, 

 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 

 the opening and a vernier of a form already described in paragraph 

 12 is arranged to slide up and down so as to enable accurate measure- 

 ments of the height of the mercurial column to be made. 



31. For marine use it is necessary 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 barome- 

 ter 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 



