218 PKOCEEDINGS OF THE AMERICAN ACADEMY 



IX. 



BRIEF CONTRIBUTIONS FROM THE PHYSICAL LABORATORY 

 OF HARVARD COLLEGE, UNDER THE DIRECTION OF PRO- 

 FESSOR JOHN TROWBRIDGE. 



No. XVI. — A NEW METHOD OF STUDYING WAVE 



MOTIONS. 



By H. H. Eustis. 



Presented May 14, 1879. 



The plan of Niemoller for interrupting in a regular manner an 

 electric circuit affords also a convenient method of studying wave 

 motion on the surface of mercury. The method of Niemoller consists 

 in providing a stretched iron wire, through which a current flows, with 

 a platinum point at its middle, which dips in mercury, and placing a 

 magnet above and near the middle of the half of the wire through 

 which the current flows. The wire, once set in vibration, is main- 

 tained in motion by the attraction of the magnet. I substituted a 

 large shallow evaporating dish filled with clean mercury for the small 

 connecting cup used by Niemoller, and an electro-magnet for his per- 

 manent magnet. Since the electro-magnet consisted of a number of 

 coarse wire coils, and formed part of the circuit with the iron wire, a 

 bright spark was produced at the break, on the surface of the mer- 

 cury. The mercury was then covered with an extremely fine layer of 

 lycopodium dust, which was effected by blowing off the lycopodium 

 after it had been sifted over the mercury. When the wire was set in 

 vibration, beautiful rings, very similar in appearance to Newton's 

 rings, emanated from the point where the break was made, and spread 

 out over the surface of the mercury. These rings were almost as 

 sharply defined as Newton's rings, and were due to the illumination of 

 the waves by the reflection of the light of the electric spark from the 

 fine particles of lycopodium dust. The regularity of the rings of 

 light made it possible to measure their radii, and in this way deter- 

 mine the rate of vibration of different lengths of wires. The follow- 



