REMARKS ON THE BAROMETER. 



681 



Barometers when in use at sea are slung in gimbals, and suspended 

 from arms at least a foot long, so as to be perfectly free to assume the 

 vertical position under every movement of the ship, and at the same time 

 to keep clear of the bulkhead against which the arm is fastened. 



Care should be takeji that no readings from a Barometer which is not 

 hanging truly vertically should ever be recorded. Such readings will 

 always be too high in proportion to the degree of obliquity. 



At Kew, all marine Barometers are rejected of which the index error at 

 the ordinary pressure is greater than 0-015 inch, or the capacity error 

 greater than 0*004 inch, or for which the mercury takes less than 3, or 

 more than 6, minutes to fall from the height of 1*5 inch, to that of 

 0-5 inch above the existing pressure. This latter condition is to ensure 

 the efificiency of the contraction as a provision against " pumping," as 

 well as to prevent the danger of the Barometer being too sluggish from 

 over-contraction . 



Correction to be applied to Barometers, with Brass Scales extending from 

 the Cistern to the top of the Column, to reduce the observation to 32° F. 



Correction for Temperature, or Reduction to 32° Fahr. — When the 

 Barometer is at a Temperature below 32°, the correction is (-f ), i.e. to be 

 added, and when it is above 32° it is (— ), i.e. to be subtracted. Thus 

 supposing the mercury in the Barometer tube to stand at 28-5 inches, and 

 the attached Thermometer registers the Temperature of the tube and case 

 (not of the surrounding air) as 50°, the correction, according to the above 

 Table, of -055 inch, must be subtracted from 28'5 inches, to reduce the 

 reading to the Standard Temperature of 32°. In taking an observation, 

 the attached Thermometer should be read first, in case of its being affected 

 by the heat of the observer's person. Brass is considered the most 

 suitable metal for Barometer cases. 



The Aneroid Barometer is an instrument which has come into extensive 

 use, owing to its convenient size and portability. In the Aneroid, atmo- 

 spheric pressure is measured by its effect in altering the shape of a small, 

 N. A. 0. 87 



