424 WRITINGS OF JOSEPH HENRY [1856 



any risk. The acid can gradually be poured into the tube 

 while in its case, slightly inclined to the horizon. Any 

 accident from breakage can be prevented by properly secur- 

 ing the whole instrument in an outer case, which will also 

 serve to equalize the temperature. To prevent the absorp- 

 tion of moisture the air may be previously passed through 

 a drying tube apparatus. The only point in which water 

 would be preferable to sulphuric acid is the less specific 

 gravity of the former, and consequently the greater range of 

 its fluctuation, which is as 20 : 11, nearly. 



The general appearance of the instrument and the several 

 contrivances for adjustment and reading are in accordance 

 with the reputation of the skillful and intelligent artizan who 

 made it. The glass tube is two hundred and forty inches 

 long and three-fourths of an inch in diameter, and is en- 

 closed in a cylindrical brass case of the same length, and 

 two and a half inches in diameter. The glass tube is secured 

 in the axis of the brass case by a number of cork collars 

 placed at intervals; which, while they prevent all lateral 

 displacement of the tube, allow it to be moved upward and 

 downward for the adjustment of the zero-point. 



The reservoir consists of a cylindrical glass bottle of four 

 inches in diameter with two openings at the top; one in the 

 axis to admit the lower end of the long tube, which is 

 tapered to about one-half of the general diameter, the other 

 to transmit the varying pressure of the atmosphere. 



To adjust the zero-point the whole glass part of the appa- 

 ratus together with the contained acid is elevated or de- 

 pressed by a screw placed under the bottom, until the level 

 of the acid in the reservoir coincides with a fixed mark. 



The scale for reading the elevation is divided into inches 

 and tenths, and by means of a vernier, moved by a rack and 

 pinion, the variations can be measured to the hundredth of 

 an inch, and estimated to a still smaller division. 



The vernier itself is not immediately attached to the 

 cylindrical brass case, but to a sliding frame which can be 

 moved along the whole opening through which the entire 

 range of the column is observed. The motion of the frame 



