SECT. XVII. SOUNDING-BOARDS. 149 



disc, by creeping along it instead of dancing up and down. If 

 the disc be made to turn round its vertical diameter while vibrat- 

 ing, the nodal lines on the paper will revolve, and exactly follow 

 the motion of the disc. It appears, from this experiment, that 

 the motions of the aerial molecules in every part of a spherical 

 wave, propagated from a vibrating body as a centre, are parallel 

 to each other, and not divergent like the radii of a circle. When 

 a slow air is played on a flute near this apparatus, each note calls 

 up a particular form in the sand, which the next note effaces, to 

 establish its own. The motion of the sand will even detect 

 sounds that are inaudible. By the vibrations of sand on a drum- 

 head the besieged have discovered the direction in which a 

 counter-mine was working. M. Savart, who made these beauti- 

 ful experiments, employed this apparatus to discover nodal lines 

 in masses of air. He found that the air of a room, when thrown 

 into undulations by the continued sound of an organ-pipe, or by 

 any other means, divides itself into masses separated by nodal 

 curves of double curvature, such as spirals, on each side of 

 which the air is in opposite states of vibration. He even traced 

 these quiescent lines going out at an open window, and for a 

 considerable distance in the open air. The sand is violently 

 agitated where the undulations of the air are greatest, and re- 

 mains at rest in the nodal lines. M. Savart observed, that when 

 he moved his head away from a quiescent line towards the right 

 the sound appeared to come from the right, and when he moved 

 it towards the left the sound seemed to come from the left, 

 because the molecules of air are in different states of motion 

 on each side of the quiescent line. 



A musical string gives a very feeble sound when vibrating 

 alone, on account of the small quantity of air set in motion ; 

 but when attached to a sounding-board, as in the harp and piano- 

 forte, it communicates its undulations to that surface, and from 

 thence to every part of the instrument ; so that the whole system 

 vibrates isochronously, and by exposing an extensive undulating 

 surface, which transmits its undulations to a great mass of air, 

 the sound is much reinforced. The intensity is greatest when 

 the vibrations of the string or sounding body are perpendicular 

 to the sounding-board, and least when they are in the same 

 plane with it. The sounding-board of the piano-forte is better 

 disposed than that of any other stringed instrument, because the 



