184 LECTURES TO SCIENCE TEACHERS. 



vibration by the body as a whole. For example, the kind 

 of motion due to the clapper of a clock bell, or that of a 

 tilt hammer. First there is a slow uniform rise, then a 

 sudden rapid fall. A violin string, bowed near one end, 

 gives a similar mode of vibration. 



Now, a body may vibrate not only in that simple form, 

 but in a more complex manner. In fact, simple motions 

 and simple waves are extremely rare. In general, com- 

 pound tones and intricate wave forms are produced by the 

 subdivisions of the vibrating body ; for a body may split 

 up into sensible parts, and these vibrate. Here, for 

 example, in this monochord we may have the wire vibrating 

 as a whole, or we may have it vibrating in a certain number 

 of aliquot parts. This also is seen most strikingly in the 

 vibrations of a plate. Here is a round plate of metal 

 which, in its simplest form of vibration, divides into four 

 equal parts ; but it may be made to subdivide itself 

 into a greater number of parts by fingering the edge of the 

 plate. Thus if I draw a fiddle-bow over the edge of the 

 plate, we get, you see, a division into a certain number 

 of vibrating parts which are seen by the motion of the 

 sand towards the nodal lines, or linos of rest. Now, such 

 subdivision gives rise to intricate wave forms, and confers 

 upon a body this peculiar quality of tone or twang which 

 enables us to distinguish the notes of different musical in- 

 struments from one another, although they may be sound- 

 ing notes of exactly similar pitch. 



So far, then, we have seen the vibration of a body giving 

 rise to sound-pulses, or motions of the air around it. But 

 a body may vibrate also in its insensible parts, and such 

 vibrations give rise to the phenomena of light, heat, and 

 possibly of electrification Here, too, in molecular, as in 

 molar motion, we may have simple and complex vibrations. 

 For example, black hot elementary gaseous bodies yield, as 

 one would expect, simpler modes of vibration than tho^e 

 corresponding to compound bodies ; and in the case of 

 solids and liquids an increasing temperature gradually 

 liberates these substances from the mutual cohesion of 

 their parts, and hence, ultimately enables them to vibrate 

 without hindrance in the definite rates peculiar to each 

 element. The body is now a glowing gas, and if in this 

 state it be examined by the prism, it is found that at an 



