CURRENT METERS 



55 



against it, so that the apparatus is locked. To release 

 the propeller, the first messenger C is allowed to slide 

 down the wire ; it forces D outwards into the position 

 shown in the figure ; the slot in E is now opposite to 



the projections, and the mechanism is freed. After 

 a measured interval a second messenger is sent down ; 

 this is larger than the first, and hollowed out below 

 so as to pass over it. It forces D still farther outwards, 

 and again locks the propeller. The direction and 



