1856] WRITINGS OF JOSEPH HENRY. 425 



enables us to make the first rough adjustment, and that of 

 the rack and pinion the minute one. 



The drying apparatus, placed between the external air 

 and the interior of the reservoir, consists of a tubulated bot- 

 tle (with two openings) containing chloride of calcium, and 

 connected with the reservoir by an India-rubber tube ; by 

 which arrangement the air is deprived of its moisture. 



To ascertain the temperature of the column of the liquid 

 two thermometers are attached, one at the top and the other 

 near the bottom. 



The whole apparatus is enclosed in an outer glazed case 

 of twelve inches square, which serves (as mentioned before) 

 as well for protection as for equalizing the temperature, 

 which is ascertained with sufficient accuracy by taking the 

 mean of the two thermometers. 



A large correction is required in this barometer for the 

 expansion and contraction by the changes of temperature. 

 To determine the amount of this, the specific gravity of a 

 quantity of the acid with which the barometer had been 

 filled was taken at different temperatures. This process was 

 performed with a very sensitive balance, by Dr. Easter, in 

 the laboratory of the Institution. 



