Francis Galton, Charles Darwin's cousin, invented the inadudible dog whistle.
The original whistle was made of a brass tube with an internal diameter of approximately 2 millimetres. By moving a small plunger, the size of the resonanting cavity in the whistle could be changed, enabling the pitch of the whistle to be altered from audible up into the ultrasonic range, which humans cannot hear but which dogs can. Galton was able to determine that the normal upper limit of human hearing was about 18kHz.
Galton, of course, also invented the field of eugenics, fingerprint indentification, and mathematical correlation and regression.
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