OF ARTS AND SCIENCES : SEPTEMBER 8, 1868. 65 



tuning-fork into the lower prong, near the extremity of which, and 

 upon the interior surface, is a small platinum plate which is touched by 

 the point of a platinum wire. This platinum wire is attached to the end 

 of a short spring. When the current begins to flow, the prongs of the 

 tuning-fork are attracted outwards, the platinum plate is withdrawn 

 from the platinum wire, the flow of the electrical current is interrupted, 

 and the prongs of the tuning-fork are free to spring together again, 

 without the retarding influence of the magnetic poles. The tuning- 

 fork itself, therefore, interrupts the current at each of its vibrations, so 

 as to be subject to an accelerating force of magnetism, when its prongs 

 are moving outwardly, without a corresponding retarding action, when 

 they are moving inwardly. 



The vibrations of this tuning-fork ai*e so energetic, and the ampli- 

 tude of its excursions is so large, that the ends of the prongs often strike 

 the poles of the electro-magnet. The tuning-fork easily commands the 

 motion of a stout cord, thirty or forty feet in length, which vibrates as 

 a whole, or in segments, whenever the tension is such as to make any 

 one of the harmonics of the cord correspond to the note of the tuning- 

 fork. The middle of the segments sweeps through a breadth of two 

 or three inches, and the eye easily recognizes the nodes, and other 

 peculiarities of vibrating cords, even when the rate of vibration is too 

 slow to produce any acoustic effect. In this way, all the laws of vi- 

 brating strings may be illustrated to the coarsest eye even more satis- 

 factorily than is possible with the most highly educated ear." 



Five Hundred and ninety-ninth Meeting. 



September 8, 1868. — Adjourned Statute Meeting. 



The Recording Secretary in the chair. 

 Professor Lovering made the following communication on 

 the Periodicity of the Aurora Borealis : ■ — 



As this paper will appear in full in the Memoirs of the Academy, 

 over three hundred pages of which are already printed, only a brief 

 abstract will be given in this place. I was incited to the study of the 

 laws of periodicity of the aurora by the absence of any recorded ap- 

 pearances of this display, in this country, before the early part of the 

 eighteenth century, — a failure in the record which could not easily be 



