Traditions and Hearthside Stories of West Cornwall, Vol. 2, by William Bottrell, , at sacred-texts.com
In old time, according to tradition, a compact cloud of mist often came in from over sea—when the weather was by no means foggy—and rested on the rocks called Cowloe, thence it spread itself, like a curtain of cloud, quite across Sennen Cove. By night a dull light was mostly seen amidst the vapour, with sparks ascending as if a fire burned within it; at the same time hooping sounds, were heard proceeding therefrom. People believed the misty cloud shrouded a spirit, which came to forewarn them of approaching storms, and that those who attempted to put to sea found an invisible force—seemingly in the mist—to resist them.
A reckless fisherman and his son, however,—disregarding the token—launched their boat and beat through the fog with a threshal (flail); they passed the cloud of mist which followed them, and neither the men, nor the Hooper, were evermore seen in Sennen Cove.
This is the only place in the west where any tradition of such a guardian spirit is preserved.