276 



METEOROLOGY. 



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[Sect. IX, 



the lower level to the fiducial point, if such be the con- 

 truction of the instrument. Then set the index to the 

 upper surface of the mercurial column^ placing the eye so 

 as to bring its back and front lower edges to coincidence, 

 and to form a tangent to the convexity of the quicksilver. 

 If the insti'ument have no tubular or double-edged index, 

 the eye must be carefully placed at the level of the upper 

 surface to destroy parallax. Whatever mode of reading 

 is adopted should be always adhered to. A magnifier 



should be used to make the contact and to read the ver- 

 nier, and the reading immediately written down and 

 careful] V entered on the register. 



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As soon after the observations have been made as cir- 

 cumstances will permitj the reading of the barometer 

 should be corrected for the relation existing between the 

 capacities of the tube and cistern (if its constructiou be 

 such as to require that correction), and for the capillary 

 action of the tube ; and then reduced to the standard 

 temperature of 32^ Fahr., and to the sea-level, if on ship- 

 board. For the first correction the jieutral point should 

 be marked upon each instrument. It is that particular 

 height which, in its construction, has been actually 2uca- 

 sured from the surface of the mercury in the cistern, and 



the scale. In general the mercury will 

 stand either above or below the neutral point ; if ahove^ a 

 portion of the mercury must have left the cistern, and 

 consequently must have lowered the surface in the cistern : 

 in this case the altitude as measured by*the scale vs^ill be 



if below. The relation of the 

 capacities of the tube and cistern should be experi- 

 mentally ascertained, and marked upon the instrument 



too short — vice 



versa 



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