What nautical tool, dating back to the days of sail, originally termed a cal, has practical uses that have diminished over time?

The boatswain's pipe, originally referred to as a cal, has significant historical importance in maritime practice. This tool was used primarily for signaling aboard sailing vessels, and it played a crucial role in communication among crew members before the advent of more modern communication technologies. The clear, sharp notes produced by the pipe could carry over long distances, allowing commands and alerts to be conveyed efficiently in noisy and chaotic conditions, such as during sailing operations or when boarding.

While the boatswain's pipe is still used today in ceremonies and traditional practices within the naval community, its practical applications have diminished considerably with the advent of radios and other electronic communication devices. As such, it serves more of a ceremonial purpose today, honoring the rich traditions of seamanship rather than functioning as a primary tool for communication on modern ships.

In contrast, the other options like flag semaphore, signal mirrors, and crow's nests have either evolved in their use or do not carry the same historical significance associated with the boatswain's pipe's early functions.