We heard the bellowing long before we came to the south foghorn on Ailsa Craig.
It obviously wasn't coming from the foghorn as it has been silent since 1966. We passed a row of nine massive but rusting compressed air tanks that provided the "oomph" for the horn. They were supplied by a 2.5" inch pipe that ran 700m from the compressor in the gasworks round to the south of the island.
The air