METEOROLOGICAL INSTRUMENTS. 



117 



the tenths into hun- 

 dredths of an inch ; 

 to the upper edge of 

 the vernier is a small 

 index which is to be 

 placed in a line with 

 the mercury ; a small 

 stud is attached to 

 move the vernier up 

 or down as occasion 

 may require ; the face 

 of the scale and ver- 

 nier are of brass, sil- 

 vered, and in the best 

 instruments they are 

 covered with glass, 

 also having rack work 

 to move the vernier up 

 and down by means 

 of a key in the front 

 of the frame below 

 the scale. Through 

 the under part of the 

 frame passes a screw, 

 with a flat round plate 

 at its end ; by turn- 

 ing the screw, the bag 

 may be so compressed 

 as to force the mer- 

 cury up to the top of 

 the tube, and also 

 to fill the reservoir, 

 which keeps it stea- 

 dy, and prevents the 

 tube from breaking, 

 by the mercury dash- 

 ing against the top, 

 when carried about. 

 The height of the 

 mercury in the tube 

 above the surface of 

 the mercury in the 

 basin is called the 



