C. GENERAL PHYSICS. 131 



slowly to and fro, in periods similar to the acoustic phe- 

 nomena known as "beats." It is evident that the number 

 of vibrations of the point is determined by the velocity of 

 rotation, the number of slits, and the duration of the beats, 

 the accuracy of the method being very great. 



In the application of his method of determining the time 

 of vibration of a tuning-fork, Poske has also been able to 

 show that the vibrations of the latter vary with the ampli- 

 tude of the arc of vibration ; that the durations diminish in 

 a geometrical series as the amplitudes diminish ; and that, 

 in general, the change in duration is proportional to the first 

 power of the amplitude, and not, as in the pendulum, in pro- 

 portion to the square of the amplitude. Poggendorff An- 

 nalen, CLIL, 463. 



THE ACTION OF ORGAX-PIPES. 



Mr. Hermann Smith states as the result of experimental 

 studies that within an organ-pipe the "air reed" vibrates 

 in arcs whose extent diminishes as we increase the speed of 

 the reed, or that the times vary with the amplitude ; and 

 to this he adds the remarkable feature that the motion of 

 vibration is an activity tempered by rests, and that the note 

 of every open organ-pipe is not single, but a concord of two 

 tones. 12 A, X., 162. 



EFFECT OX SOUXD AXD LIGHT OF THE MOVEMENT OF THE 



OBSERVER, 







The long-vexed question as to the effect, upon observa- 

 tions, of the movement of the observer, and the source of 

 light or sound, has been elucidated by Baron Eotvos, of 

 Pesth, who, in a recent communication, extends his former 

 investigations, and offers a satisfactory refutation of several 

 objections that have been raised. According to him, in 

 case the source of sound or light be moving, the intensity 

 must be defined as the living force that would fall, in a unit 

 of time, upon a unit of surface, parallel to the wave surface, 

 if all vibrations were like those which are imparted to the 

 surface at that instant in which the intensity is to be de- 

 termined. The formula for the intensity in question, as 

 deduced by Eotvos, shows that the movement of the observer 

 has a decided effect upon the result ; and by applying this 



