6 ART. 10. S. KUSAKABE. 



and is maintained by the continued operation of that force, it 

 is obviously a forced vibration. It must, however, be remembered 

 that any free vibration which we shall have in a laboratory ex- 

 periment takes its origin necessarily from a force acting upon it 

 from without. At first, there is a forced vibration not less im- 

 portant than its rival, but when the force is removed, though 

 there is no discontinuity in velocity or displacement, yet the 

 period of the force is at once exchanged for that natural to 

 the system and the forced vibration is converted into a free 

 vibration. 



In the present case, the frequency of the hammer-blow, 

 which is easily adjustable weithin wade limits by varying its 

 moment of inertia, is about ten per second ; while that of the 

 vibration of the specimen lies between some three hundreds and 

 a thousand per second. In other words, between two consecutive 

 tappings, the number of vibrations of the specimen amounts to 

 some thirty or one hundred, so that the mode of vibration is 

 necessarily of a free nature. 



in. TEST OF THE APPARATUS WITH A TUNING-FORK 

 AND A PRISM OF SOFT IRON. 



So far as the principle is concerned, the above statements 

 are sufficient and nothing more is required. In laboratory work, 

 however, there are several experimental difficulties to be overcome ; 

 especially, maintenance of the vibration and the fulfilment of 

 the condition of a clamped end. The fixedness of the clamped 

 end is absolutely important, since the frequency varies inversely 

 as square of the length of the vibrating portion. 



