988 



SCIENCE. 



[N. S. Vol. XIV. No. 365. 



sooner because of the cooperation which 

 physicists may naturally look for from those 

 who are cultivating the new fields of ex- 

 perimental psychology. 



In order to avoid the tedium of a bare 

 enumeration of discoveries arranged chron- 

 ologically, I propose to refer in the first 

 instance to the invention of the various ex- 

 perimental methods which have been em- 

 ployed in acoustical research. A separate 

 reference to these methods will enable us to 

 appreciate their potency in the advance- 

 ment of this science. 



The earliest of these methods is due to 

 Chladni, whose work, * Die Akustik,' ap- 

 peared in the form of a French translation 

 in 1809, under the title ' Traite d'Acoustique 

 de Chladni.' In this work were collected 

 all the researches on the vibrations of bodies 

 which Chladni had conducted with the 

 aid of the new method (m^thode de sable). 

 This method consists in distinguishing, on 

 the surfaces of vibrating bodies, the parts 

 which are vibrating from the parts which 

 are in repose, by means of the sand which is 

 driven from the former to collect on the lat- 

 ter. In these experiments of Chladni on 

 plates, etc., the violin bow was used for the 

 first time to produce the necessary vibra- 

 tions. The bow had previously been used 

 only for vibrating cords, the ' violon de fer,' 

 and other musical instruments. Chladni 

 made his discovery of sand figures in 1787, 

 having been led thereto by Lichtenberg's 

 discovery of electric figures. 



The transversal nodal lines given by 

 Chladni's method in the case of rods vibrat- 

 ing longitudinally were readily explained. 

 Not so, however, the complicated nodal 

 lines presented by vibrating plates, or the 

 alternate lines which appear on the two 

 sides of rods vibrating longitudinally, and 

 which sometimes also appear on rods vi- 

 brating transversally. It was not until 

 1833 that an explanation of the former of 

 these phenomena was offered by Wheat- 



stone's theory that the nodal lines were due 

 to the superposition of transversal vibra- 

 tions, corresponding to sounds of the same 

 pitch coexisting with respect to different 

 directions in the plate. This theory was 

 confirmed experimentally in 1864 by Eu- 

 dolph Koenig, who constructed rectangular 

 plates giving unison notes corresponding to 

 different sets of nodal lines parallel to two 

 adjacent sides of the plate. The theoretical 

 figure results when the plate is vibrated so 

 as to produce the coexisting unison notes. 



The alternate nodal lines given by vi- 

 brating rods were also explained by the 

 theory of the coexistence of two sounds 

 near unison in the same vibrating rod. In 

 this case, however, one sound corresponds 

 to longitudinal, and the other to transversal 

 vibrations. This explanation was first 

 given by Augusta Seebeck in 1849, whose 

 theory was confirmed in 1859 by Terquem 

 in a very important paper ' Sur les vibra- 

 tions longitudinales des verges libres aux 

 deux extremites.' 



In 1807, five years after the publica- 

 tion of Chladni's ' Akustik,' appeared Dr. 

 Thomas Young's ' Course of Lectures on 

 Natural Philosophy and the Mechanical 

 Arts ' in which we find the earliest descrip- 

 tion of the graphical method, including its 

 application to chronography. This descrip- 

 tion is as follows : 



"By means of this instrument we may 

 measure, without difficulty, the frequency 

 of the vibrations of sounding bodies, by 

 connecting them with a point which will 

 describe an undulated path on the roller. 

 These vibrations may also serve in a very 

 simple manner for the measurement of the 

 minutest intervals of time ; for if a body, of 

 which the vibrations are of a certain degree 

 of' frequency, be caused to vibrate during 

 the revolution of an axis, and to mark its 

 vibrations on a roller, the traces will serve 

 as a correct index of the time occupied 

 by any part of the revolution, and the motion 



