144 NODAL LINES IN AtR. SECT. XVII. 



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 

 center, 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 estab- 

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

 Bounds 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 beautiful experiments, employed this appa- 

 ratus 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 remains at rest in the nodal 

 lines. M. Savart observed, that when he moved his 

 head away from a quiescent line toward the right the 

 sound appeared to come from the right, and when he 

 moved it toward 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 vi- 

 brating 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 undula- 

 tions to that surface, and from thence to every part of 

 the instrument ; so that the whole system vibrates iso- 

 chronously, and by exposing an extensive undulating sur- 

 face, 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 sound- 

 ing-board of the piano-forte is better disposed than that 

 of any other stringed instrument, because the hammers 



