334 EXPERIMENTS ON TRANSMISSION OF SOUND. 



but not very small board called a sounding -board, which 

 in consequence of its comparatively large surface am 

 great elasticity is peculiarly capable of receiving so 

 norous vibrations from the air, and, conversely, of com 

 nmnicating its own vibrations to the air. With sound 

 ing-boards of three or four square decimetres area 

 and a length of about 100 m of twine stretched betweei 

 them, the slightest tapping of a pencil or the finger upoi 

 the board at one end can be distinctly heard at the othe 

 end. A musical-box, set playing upon one board, i 

 heard at the opposite as distinctly as if it were clos 

 to the ; ear 5 . 'Words pronounced in a low voice at a di^ 

 tance of 10, cm ,from one board are perfectly audible t 

 an ear placed near the board at the other end. 



Jf an iron wire, about O mm *6 thick, be very tightl 

 stretched between two sounding-boards, it is possibl 

 to transmit the gentlest knocking of the finger, or th 

 sound of words spoken moderately low, over a distant 

 of more than 600 m . If several persons transmit wore 

 in this manner, the difference in their voices is distinct! 

 recognised. A short sharp cry produced within 1 

 from the board is heard twice at the other end : tl 

 first being due to the transmission of the sound by tl 

 wire ; the second, which arrives a little later, is tran 

 mitted' by the air, the propagation through air takin 

 place more slowly than that through the wire. 



Common wooden cigar-boxes having a size of about 25 cm in lengt 

 14 cm in breadth, and 8 cm in height, may be used as soundin 

 boards. The top is removed, and the four sides left to make t 

 thin bottom firmer. Better still are sounding-boards made of venet 

 of pine- wood, 1*5 or 2 mm thick, glued to frames which are abo 

 20 cm long and 2 cm high ; the sides of the frames, which are be 

 made by a joiner, should have a thickness of about 6 or 8 mm . 



