PHILOSOPHICAL SOCIETY OF WASHINGTON. 157 



interrupting disk (C.) The two substances, whose sonorous powers 

 are to be compared, are placed in the receiving vessels (D E) (so 

 arranged as to expose equal surfaces to the action of the beam) 

 which communicate by flexible tubes (F G) of equal length, with 

 the common hearing-tube (H.) The receivers (D E) are placed 

 upon slides, which can be moved along tbjS graduated supports (I 

 K.) The beams of light passing through the interrupting disk (C) 

 are alternately cut off by the swinging of a pendulum, (L.) Thus 

 a musical tone is produced alternately from the substance in D and 

 from that in E. One of the receivers is kept at a constant point 

 upon its scale, and the other receiver is moved towards or from the 

 focus of its beam until the ear decides that the sounds produced 

 from D and E are of equal intensity. The relative positions of the 

 receivers are then noted. 



(2.) Another method of investigation is based upon the produc- 

 tion of an interference of sound, and the apparatus employed is 

 shown in Fig. 11. The interrupter consists of a tuning-fork, (A,) 

 Avhich is kept in continuous vibration by means of an electro- 

 magnet, (B.) 



A powerful beam of light is brought to a focus between the 

 prongs of the tuning-fork, (A,) and the passage of the beam is 

 more or less obstructed by the vibration of the opaque screens (C 

 D) carried by the prongs of the fork. 



As the tuning-fork (A) produces a sound by its own vibration, it 

 is placed at a sufficient distance away to be inaudible through the 

 air, and a system of lenses is employed for the purpose of bringing 

 the undulating beam of light to the receiving lens (E) with as little 

 loss as possible. The two receivers (F G) are attached to slides 

 (H I) which move upon opposite sides of the axis of the beam, and 

 the receivers are connected by flexible tubes of unequal length (K 

 L) communicating with the common hearing-tube (M.) 



The length of the tube (K) is such that the sonorous vibrations 

 from the receivers (F G) reach the common hearing-tube (M) in 

 opposite phases. Under these circumstances silence is produced 

 when the vibrations in the receivers (F G) are of equal intensity. 

 When the intensities are unequal, a residual effect is perceived. In 

 operating the instrument the position of the receiver (G) remains 

 constant, and the receiver (F) is moved to or from the focus of the 

 beam until complete silence is produced. The relative positions of 

 the two receivers are then noted. 



