SECT, xvii.] RESONANCE. 167 



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 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 ex- 

 tensive undulating surface, which transmits its undulations 

 to a great mass of air, the sound is much reinforced. The in- 

 tensity is greatest when the vibrations of the string or sound- 

 ing 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 hammers strike the 

 strings so as to make them vibrate at right angles to it. In 

 the guitar, on the contrary, they are struck obliquely, which 

 renders the tone feeble, unless when the sides, which also act 

 as a sounding-board, are deep. It is evident that the sound- 

 ing-board and the whole instrument are agitated at once by 

 all the superposed vibrations excited by the simultaneous or 

 consecutive notes that are sounded, each having its perfect 

 effect independently of the rest. A sounding-board not only 

 reciprocates the different degrees of pitch, but all the nameless 



