The mystery explained
Thursday 6 June 2013
On one occasion, whilst examining the machinery of the monster revolving lamp belonging to a lighthouse, a visitor, wishing to see how many seconds would elapse before it completed a revolution, took a half-crown from his pocket, and placed it on the revolving framework.
Watch in hand, he patiently waited for the coin to come round again to where he was standing, but no half-crown appeared. The seconds lengthened out into minutes—still no half-crown.
“Strange!” he exclaimed. “What can be the reason of it?”
In order to find out, he walked to the other side of the lamp, and met one of the lighthouse men, who touched his cap and said respectfully,
“Thank you, sir.”
[from the Rockhampton newspaper The Capricornian, 14 December 1907.]
The 1st order optical apparatus at Rottnest Island Lighthouse, installed when the lighthouse was built in 1895, and still in service. I was there in April 2007 and I can tell you, gratis, that it takes 60 seconds to complete a revolution. The eight-sided lens assembly floats in a trough filled with mercury and, at night, shows a brief flash of light every 7.5 seconds to vessels sailing within 23 nautical miles of the lighthouse.