460 



PHYSICAL SCIENCES. 



NOTE 179, p. 137. Reflected waves. A series of waA-es of light, sound, 

 cr water, diverge in all directions from their origin I, fig. 43, as from a 



Fig. 43. 



centre. When they meet with an obstacle S S, they strike against it, and 

 are reflected or turned back by it in the same form as if they had pro- 

 ceeded from the centre C, at an equal distance on the other side of the 

 surface S S. 



NOTE 180, p. 138. Elliptical shell. If fig. 6 be a section of an ellip- 

 tical shell, then all sounds coming from the focus S to different points on 

 the surface, as m, are reflected back to F, because the angle T m S is equal 

 to t m F. In a spherical hollow shell, a sound diverging from the centre is 

 reflected back to the centre again. 



NOTE 181, p. 142. Fig 44 represents musical strings in vibration ; 

 the straight lines are the strings when at rest. The first figure of the four 



Fig. 44. 



...;.. 



would give the fundamental note, as, for example, the low C. The second 

 and third figures would give the first and second harmonics ; that is, the 

 octave and the 12th above C, n n n being the points at rest ; the fourth 

 figure shows the real motion when compounded of all three. 



