TRIGGER IMBEDDED IN RUBBER 

 TO ALLOW DENTED STRIP OF 

 STEEL TO BE BENT- 



^ 



Fig. 7. A very satisfactory device for echo experiments 

 can be made like this. The inside of the horn should 

 be a paraboloid of revolution, and the clicker must be 

 mounted at the focal point of the parabola. The Fiber- 

 glas and plastic boat- or car-patching materials laid on 

 a plaster of Paris form make excellent horns, and so 

 do the parabolic reflectors of certain desk lamps. 



68 



