18 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



it is necessary that the thread fixed at o and stretched by a weight 

 should coincide when at rest with the test-line of the rule. In 

 order that the test be complete, the straight-edge ought to be reversed 

 and the same experiment made with the side c r>. 



I 



Fiu. 2. Masons', or perpendicular levels. 



The levels shown in Fig. 2 are used to prove that a plane or a line 

 is horizontal. The appearance of the instruments as sufficient to in- 

 dicate the way in which they are employed, and we need not dwell 

 longer on this simple application of the first law of gravity, which 

 teaches us that its direction is constant in one place. 



Vm. 3 - Delambre's perpendicular level for geodetic observations. 



In geodesy, the perpendicular level (the name given to the instru- 

 ments represented in Figs. 2 and 3), made with the greatest accuracy, 

 is used to measure the angle of inclination of a straight line to the 

 horizon. The plumb-line is replaced by a heavy rod suspended at o ; 

 the lower extremity of which is furnished with a vernier. A graduated 

 limb gives in degrees the value of the angle (p o E) formed by the 

 level and the test-line. The inclination of a line (A B) to the horizon 

 (A H) can thus be found; FOR is, in fact, equal to the angle B A ir, 

 as the two sides of these two angles are perpendicular to each other. 



Delambre, in his measurements of the meridian, used a perpen- 

 dicular level thus arranged, in order to determine the inclination to 



