š¤ According to the text, what is the primary sense that fog signals appeal to?
Difficulty: Easy
š§ According to the text, what were once considered serious obstructions to sound, but have since been scientifically proven to have no sensible power to obstruct sound?
Difficulty: Medium
𤯠What does the text suggest is the true test of a sound signal's effectiveness, rather than how far it can be heard under ideal conditions?
Difficulty: Hard
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