24 s. Xakamiira and K. Houda : 



projected a little from the table, two nails were fixed, from which 

 a leaden ball weighing about 2 kilograms was hung. When the 

 ball was made to swing, the impulse was imparted to the water 

 and it was forced to oscillate with the pendulum. The experiment 

 was not so simple however as it appears at first sight. For when 

 the length of the pendulum was so far adjusted that its period 

 ■came very neai- to the natural period of the model, and when the 

 water was apparently moving in harmony with the pendulum, it 

 was impossible to say Avhether the imposed period was the natural 

 period of the model or an enforced period differing slightly from 

 it. One can however decide this point more or less accurately 

 by stopping the ball suddenly and thus removing the force 

 ■acting on the water; for when the enforced period was equal 

 to the natural period, the movement of the water would go on 

 anclianged and gradually die away, but when the periods did 

 not coincide, the water would soon assume a new phase of 

 oscillation and so come to rest. Thus our experiments with the 

 models were conducted as follows. The natural period was 

 determined approximately by means of a stop watch, when the 

 model was moved by hand and left to itself witli no pendulum 

 attached to it. Then the length of the pendulum was adjusted till 

 its period coincided with the approximate peiiod found as above. 

 In general, when any two given oscillating systems are made to 

 move together, their motions are influenced reciprocally, producing 

 an oscillasion with a peiied differing from the free periods of the 

 two systems. So here when the model lake and the pendulum are 

 made to move togetliei-, tlie actual period was nearly but not quite 

 e(iual to the prescribed period. The period was determined afresh, 

 and tlio motion was attentively watched to see whether the 

 oscillation continued unchanged in its ])hase and whether the 



