54 



NINTH REPORT. 



FlOURE 1. 



and disappears. The first mentioned condition is re}:) resented in the ex]:)cri- 

 ment l)y those portions of the bridge where l)locks are shallowest, and where 

 in consequence they are suddenly dropped when the compression is relieved. 

 If the entire bridge be depressed in such a manner that the water overflows 

 it before the experiment, the opposite condition may be locally illustrated. 

 Whenever two surfaces slide over each other under pressure they tend to 

 assume a more or less interrupted but progressive movement as a resiilt of 

 friction; and this alternation of quicker and slower motion is transmitted 

 outward in all directions as elastic vibrations or Avaves, whenever the moving 

 bodies are in contact with an elastic medium. The waves will be of the 

 greater intensity according as the slips are the larger — have greater 



FiGUUF. 2. 



