THE PLUMB-BOB AND THE PENDULUM 167 



64. The Plumb-bob and the Pendulum. 



Since the force of gravity acts toward the center of 

 the earth, and since a tight string makes a straight line, 

 we can easily obtain a line, which if con- 

 tinued would pass to the center of the 

 Dearth, by hanging a weight on the end of 

 a string. This is called a plumb-bob and 

 the direction of the line is called vertical. 

 The name "plumb" comes from the Latin 

 and means lead. This was the best mate- 

 rial for plumb-bobs in olden times. We 

 should make the wall of our buildings ver- 

 tical and we can do it by using the simple 

 method which is shown in the illustration. 

 As you have learned, the surface of 

 water is level. Another name for level is 

 horizontal. How many degrees are there 

 between a horizontal line and a vertical 

 line? Float a piece of wood upon some 

 water in a dish and hang a plumb-bob in 



the water, as shown in 

 the illustration. Then 

 measure the number 

 of degrees there are 

 between the direction 

 of the wood and the di- 

 rection of the plumb- 

 line. See Section 81. 

 If we swing our 

 plumb-bob we have a 

 pendulum. You read 

 in Section 8 that the 



