Two Harmonic Analyzers. 227 



two feet long, a cross notch being cut in one end to enable 

 this end to be held against any point of the structure with 

 less chance of slipping. About four inches from the 

 notched end, right across the axis of this bar, is a hole, in 

 which is fitted, with moderate tightness, a piece of straight 

 steel wire, one-eighth of an inch in diameter, and 18 inches 

 long. On one end of the wire is a ball of lead, about 2 oz., 

 through the centre of which is a small hole at right angles 

 to the wire, in which is fixed a small graphite pencil. On 

 the other end of the wire is a carrier, to afford handhold for 

 the purpose of adjusting the wire in the hole. 



When the carrier is pushed right up to the wood, the 

 ball, if disturbed, will vibrate in any direction perpendicular 

 to the wire so as to make about 200 oscillations a minute, 

 which is slower than any period it is required to measure. 

 As the carrier is pulled back, and the wire between the base 

 and the ball shortened, the rate of vibration increases, until, 

 when the wire is only i J^ inches long, the ball, when dis- 

 turbed, gives out an audible note of about 2,000 vibrations a 

 minute. 



The instrument is used by holding in one hand the 

 longer end of the wood and pressing the notched end hard 

 against the point of the structure of which the motion is to 

 be analyzed, the carrier having previously been pushed up 

 to the wood, then, with the free hand, the carrier is pulled 

 steadily back, the ball being carefully watched. As by the 

 shortening of the wire between the base and the ball the 

 free period of vibration of the ball is diminished, and comes 

 near to any period amongst the vibrations in the structure^ 

 the ball is seen to take up the vibration in beats with 

 intervals of rest ; and a very little more careful adjustment 

 is sufficient to bring the period into coincidence, when the 

 ball continues vibrating with the structure, having the 

 appearance in Fig. 2. 



