SURVEYING 



45 



Fig. 



33. A carpenter's level 

 sights attached. 



with 



or marked to indicate clearly the position of the bubble 

 in the tube. 



If the inside of the bubble tube is truly circular length- 

 wise, then as the bubble tube is held so as to bring the con- 

 vex side of the tube up, it is plain that the bubble will come 

 to the highest point. This being the case, a line tangent to 

 the curvature of the tube at this point is a level line regard- 

 less of the part of the tube in which the bubble may lie. 



If the bubble tube is attached to a frame and placed on 

 two supports and one of these supports is raised or lowered 



until, as the frame is 

 reversed on the supports, the 

 bubble will occupy the same 

 position, these supports are 

 both in a level line, provid- 

 ing the identical points in 

 the frame come in contact with the supports in each case. 

 Furthermore, the points on the frame will be in a level line 

 when the bubble is brought into the position described. 



Thus the carpenter's level, used for leveling buildings, is 

 made. If sights are provided on the level, the level line so 

 obtained may be extended to a greater distance. A line 

 tangent to the bubble tube 

 on its inner surface at its 

 center as indicated by the 

 marks on the tube is known 

 as the bubble axis. If the 

 bubble tube be revolved 

 about a line perpendicular 

 to the bubble axis, the bub- 

 ble axis will describe a level 

 surface. 



The Level. The instrument used generally by engineers 



Fig. 34. An inexpensive farm level 

 with horizontal circle for turning off 

 angles. 



