20 



Prof. Barton and Miss Browning on 



The first method adopted for suspending three inter- 

 connected pendulums was shown in figs. 1 and 2 of paper I. 

 (Phil. Mag. Nov. 1920). The present simpler arrangement 

 differs from that just referred to in that the suspension 



Fig. 4. 

 Oblique Plan. 



points on the bridle cords are now all in one horizontal 

 straight line, instead of the centre point being lower and 

 advanced towards one end. We also now usually restrict 

 ourselves to two bridges, BB' and C'C", which is understood 

 to be the case unless anything to the contrary is specified. 



It may be seen from the figures, that if any one of the 

 three pendulum bobs P, Q, R is displaced and let go (or 

 started in any other manner), each of the other pendulums 

 will respond accordingly. The nature of such response (in 

 both qualitative and quantitative aspects) is calculable and 

 capable of ready verification by means of the sand traces left 

 from the funnels on the moving board. 



III. Theory of Triple Pendulums. 



(a) Pendulum Lengths and Relations among them. 



In the first paper it was pointed oat that if a bridge 

 (BB ; , say) were held still while one of the corresponding- 

 pendulums was isolated and started executing small vibra- 

 tions, its virtual length would be given by a simple 

 geometrical construction. This length is now called b, and 

 shown in figs. 1 and 2. The length in question is seen to be 

 the vertical depth of the bob P below the point B on the 

 dotted line BE. Similarly, for the bridge C'C", the corre- 

 sponding depth of the bob P below O on the dotted line 

 C'E" is called c, and is the virtual length of this pendulum 



