392 Mr Russell on the Vibration of 



timed force, like that of soldiers marching in ranks, is the 

 greatest trial to which such a bridge can be subjected. A sus- 

 pension bridge near Manchester was destroyed by this means, 

 and it has, therefore, been necessary for the curators of such 

 bridges, to order that soldiers, in crossing such bridges, shall 

 walk with unequal paces instead of the usual military march. 



In observing the vibrations of a suspension bridge, it will 

 be noticed that it divides into nearly equal portions which 

 oscillate in nearly equal times — figure 9 will give an idea of this 



Fig. 9. 



vibration — when one-half of the bridge is falling the other is 

 rising, it remains nearly stationary in the middle vibrating, in 

 halves ; again it will be noticed at another time vibrating, in 

 three parts as in fig. 10, or in four parts as in fig. 11, and so on. 



Fig. 10. 



Now, in order to prevent these oscillations, it will be of no 



