*78 PLEASANT WA YS IN SCIENCE. 



moving in vibrations of a special kind by the sounds 01 

 words uttered, and the disc or diaphragm responds to these 

 vibrations. Nevertheless, though it is important that this dis- 

 tinction be recognized, we can still learn, from the behaviour 

 of discs and plates set in vibration by a blow or other im- 

 pulse, the laws according to which the actual motions of the 

 various parts of a vibrating disc or plate take place. We 

 owe to Chladni the invention of a method for rendering 

 visible the nature of such motions. 



Certain electrical experiments of Lichtenberg suggested 

 to Chladni the idea of scattering fine sand over the plate or 

 disc whose motions he wished to examine. If a horizontal 

 plate covered with fine sand is set in vibration, those parts 

 which move upwards and downwards scatter the sand from 

 their neighbourhood, while on those points which undergo 

 no change of position the sand will remain. Such points are 

 called nodes ; and rows of such points are called nodal lines, 

 which may be either straight or curved, according to circum- 

 stances. 



If a square plate of glass is held by a suitable clamp at 

 its centre, and the middle point of a side is touched while 

 a bow is drawn across the edge near a corner, the sand is 

 seen to gather in the form of a cross dividing the square into 

 four equal squares like a cross of St. George. If the 

 finger touches a corner, and the bow is drawn across the 

 middle of a side, the sand forms a cross dividing the* square 

 along its diagonals like a cross of SL Andrew. Touching 

 two points equidistant from two corners, and drawing the 

 bow along the middle of the opposite edge, we get the 

 diagonal cross and also certain curved lines of sand 

 systematically placed in each of the four quarters into which 

 the diagonals divide the square. We also have, in this case, 

 a far shriller note from the vibrating plate. And so, by 

 various changes in the position of the points clamped by the 

 finger and of the part of the edge along which the bow is 

 drawn, we can obtain innumerable varieties of nodal lines 

 and curves along which the sand gathers upon the surface of 

 the vibrating plate. 



