422 



GENERAL SCIENCE 



having tonal value. In this wood are little cup-shaped discs. In 

 the center of each of these discs is a small membrane (I) similar to 

 the membrane of the ear or phonograph sound box, having a thick- 

 ened part (pi) in the center. It is this vibrating membrane that 

 gives tonal value to the wood. There are millions of these mem- 

 branes in every square inch of pine or spruce. 



It is exceedingly important that a pine or spruce tree after being 

 cut down be properly cared for. It must be air seasoned. If the 

 delicate membranes of the wood are ruptured through carelessly 

 kiln drying it, the tonal values will be destroyed. 



FIG. 309. Cells in wood used for a musical instrument magnified 250,000 times. 



Simple and Compound Tones. Simple tones are produced by a 

 body vibrating as a whole. 



In the illustration, Fig. 310, a diagram of sound waves is shown 

 produced by a tuning fork vibrating as a whole, producing a funda- 

 mental. Piano strings not only vibrate as a whole, but in parts. 

 Fig. 311 shows the string of a violin vibrating as a whole, and 

 vibrating in many parts at the same time. The vibration of the 

 whole string produces the fundamental tone which determines 

 the pitch. The many little vibrations of the string produce tones 

 above the fundamental which are called partials or overtones. Fig. 

 312 shows sounds produced by vibrations as a whole and in many 

 parts. The partials or overtones give tone quality and richness. 



A heavy bass piano string about 10 feet in length may be obtained from the 

 American Steel and Wire Corporation. This will show the vibrations of a 

 piano string and its fundamentals and overtones plainly enough to be seen by 

 every member of the class. 



