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TABLE XVIII. 



FOR REDUCING THE INDICATIONS OF ENGLISH BAROMETERS, WITH WOODEN OR 

 GLASS SCALES, TO THE FREEZING POINT. 



IN most of the common barometers the scale is engraved upon a short plate of 

 brass, or of ivory, fixed upon the wooden frame of the instrument. In such a case, 

 tne compound expansion of the two substances can only be guessed at, and the cor- 

 rection to be applied to the observations for reducing them to the freezing point can- 

 not be determined with precision. As a near approximation for such imperfect 

 instruments, the following table may be used. In computing this table, the expansion 

 of glass, which is less than that of brass and greater than that of wood, has been 

 substituted for that of brass, as an approximate value for a scale composed of these 

 last two substances. The table thus gives the true correction, in English inches, 

 for the barometers, the graduation of which is engraved on the glass tube itself. It 

 answers equally for any English barometer with wooden scale, whatever be the sub- 

 stance of which the short plate bearing the graduation is made. 



CORRECTIONS TO BE APPLIED TO ENGLISH BAROMETERS, WITH WOODEN OR GLASS 

 SCALES, TO REDUCE THE OBSERVATIONS TO THE FREEZING POINT. 



Expansion of Mercury for 1 Fahr. = 0.0001001 ; of Glass for 1 Fahr. = O.OOOOOM4. 



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