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 o 

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

 the compound expansion of the two substances can only be guessed at, and the coi 

 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 "beeni 

 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 j 

 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. = 0.00000444. 



