CHAP, i.] DIRECTION OF GRAVITY 17 



CHAPTER I. 



DIRECTION OF GRAVITY FALL OF BODIES OSCILLATIONS OF THE 



PENDULUM. 



I. PLUMB-LINE AND LEVELS. 



IN the arts, and especially in the art of building, it is frequently 

 necessary to establish vertical or horizontal lines or planes ; or, 

 if these lines or planes are already constructed, it becomes equally 

 important to test their accuracy. This is done by means of instru- 

 ments called plumb-lines or *levels, both based on the fact that a 

 thread or string stretched by a heavy body lies, when at rest, 

 in the exact vertical of the place where the observation 

 is made. 



Most people have seen the plumb-line used by 

 masons, which consists of a thread, with a cylindrical 

 metal weight attached, and a square plate, also of metal, 

 the side of which is equal to the diameter of the cylinder. 

 The plate slides, by means of a central hole, along the 

 thread and is placed against the wall, the vertically oi 

 which is to be observed. When not in motion, the cylinder 

 should lie along the surface of the wall, without resting 

 against it and without leaving between it and the wall 

 any perceptible interval. 



A flat rule or straight-edge (Fig. 1), having truly parallel 

 edges (A B, c D), with a straight line (o i) drawn down the centre, 

 called a test-line, is also used for the same purpose. One of 

 the sides (A B) is placed against the line or plane to be tested, and 



c 



