LEVELS. 



159 



tself. This cannot happen, as will be seen sub- 

 sequently, unless the surface of the Vater is at right 

 ingles with the thread. But the dir^ct^on of the thread 

 s vertical, and the direction perjgejaaicular to it is hori- 

 ;ontal: the surface of the liquid is hence horizontal. 1 



It follows from the action of gravity upon a liquid 

 yhich occupies only part of a vessel, that the unoccupied 

 pace will be above the surface of the liquid, that is, in 



FIG. Ill (i real size). 



e highest part of the vessel, whatever position the 

 Jtter may assume. This fact is applied in instruments 

 uployedfor determining a line or surface which is per- 

 iptly level or horizontal. These instruments are called 

 Ms ; figure 111 represents a section of a kind of level, 

 i ually called a spirit-level, in three different positions. 

 r ie spirit-level consists of a glass tube very slightly 

 crved, filled with spirit with the exception of a small 

 Slice containing air which tends to occupy the highest 

 l-'rt. This tube is placed in an outer tube of brass 



The surface of a liquid is generally not horizontal where it is in contact 

 wh the vessel. This exception from the law will be considered in art. 23. 



