From: Plymouth and South-West Devon. Ward-Lock & Cos. Illustrated Guide Books, 1913.
The distance between Plymouth Sound and London is 239 miles. It is amazing to consider that semaphore messages could be sent and returned within 20 minutes between these two locations!
Initially commissioned as HMS Howe in 1860, HMS Impregnable was a 121-gun ship that was already obsolete at the time of her launching by the first ironclad battleships. Consequently, she was converted to a training ship in 1886. She was broken up in 1921. She would have been quite a sight to see, berthed in Plymouth Sound. Her timbers are now used decoratively in the high-end store, Liberty, on Regent Street in London.