160 LEVELS. 



which has an aperture in the middle of the upper side. 

 and when the instrument is in a perfectly horizontal 

 position the bubble of air is exactly in the middle ot 

 the aperture ; the exact position of the bubble in that case 

 being usually marked by lines etched upon the glass tube. 

 The brass tube is fixed upon a straight ruler of brass 01 

 iron. In determining a horizontal line one end of the 

 level is raised or lowered until the bubble is in the 

 middle, as shown in fig. Ill A. But it may happen, a^ 

 in the case shown at B, that the level itself is not perfect!} 

 adjusted, and that the bubble is in the middle, althougl 

 the line is not horizontal. This error may be recognisec 

 by reversing the level, so that the end which was befon 



FIG. 112 (\realsize). 



on the right side may now be on the left : if the level i 

 correct, the bubble will settle in the middle as before 

 but if not, it will move to the side which is highest, a 

 shown in C . Good levels are usually provided wit) 

 screws for correcting this error by raising or lowerin: 

 permanently one end of the level. Another form c 

 level is shown in fig. 112. It consists of a flat box cj 

 brass, with plane sides ; the top is of glass, flat outsid 

 but concave inside, the radius of the concave surfac 

 being from O m '5 to l m , while in the spirit-levels pr( 

 viously described the tube forms an arc of a circle < 

 which the diameter varies from 2 to 10 m . A hoi 

 closed by the screw s serves for filling the level wit 

 spirit. 



