42 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



The line of sight along the two surfaces of the water in the vessels, 

 provided that the diameters of the glass tubes be exactly the same, 

 will be horizontal. By turning the instrument on its axis in another 

 direction, the new line of sight will be likewise horizontal and in the 

 same plane as the first. 



By a series of experiments, which it is not necessary to describe 

 here, those portions of land-surface on the same level can be deter- 

 mined ; in other words, contour lines can be drawn with great accuracy 

 and rapidity. 



FIG. 20. Water leve 



Spirit-levels, like water-levels, are used to determine the hori- 

 zontality of a line or plane; but their construction is based on a 

 different physical principle. 



Imagine a' closed glass tube in a metal mounting, which leaves a 

 part of the tube visible (Fig. 21). It is entirely filled with a liquid 

 water, alcohol, or ether (these last are preferable to water, because 



FIG. 21. Spirit-level. 



they do not freeze) with the exception of a very small space filled 

 with a bubble of air or vapour. By virtue of the law of equilibrium 

 of fluids of different density, the gaseous bubble will always be found 



