Solitary travellers long ago were sometimes lured to their death as they followed a dancing light across a treacherous stretch of bog. It was said of it that it "leadeth men up and downe in a circle of absurditie, so they never knowe where they may be".
And Hiawatha had to contend with it in the course of his hazardous journey through the swamp to slay the mad magician; the evil mire through which he paddled was
. . . rank with flags and leaves of lilies,
Stagnant, lifeless, dreary, dismal,
Lighted by the shimmering moonlight,
And by will-o'-wisps illumined
Fires by ghosts of dead men kindled,
In their weary night encampments.
In these last two lines Longfellow hints at the most likely explanation for the will o' the wisp, or the ignis fatuus, as it was sometimes called. This strange phenomenon is believed to be caused by the spontaneous combustion of certain gases, probably methane, resulting from animal, vegetable or indeed human decomposition.
The ignis fatuus, literally "the foolish fire", was observed more often in times gone by than it is now. It is a tiny brightly coloured flame a few inches high, commonly seen in bogs and graveyards and over marshy ground, and often described as resembling a lantern being carried by a person moving in a zigzag line, but it has a multiplicity of other names as well.
Liam an tSoip, for example, as we knew it here in Ireland, is a straight translation. Jack o' Lantern, Friar's Lanthorn, and Peg a' Lantern, on the other hand, derive from its behaviour, as do Kit o' the candlestick and walking fire. In some traditions, as in the native American culture portrayed in The Song of Hiawatha, the dancing flames were thought of as the spirits of the dead, flitting between the upper and the lower worlds; indeed this may explain why they were sometimes called "corpse candles".
The phenomenon also has names like Spunkie, Gillian burnt-tail, John in the Wad, Dick a' Tuesday, and Sylam Lamps, whose etymology is lost in local folklore. But names like Elf-fire, Robin Goodfellow and Hobgoblin reflect the supposedly mischievous nature of the apparition.
The poet Drayton, for example, tells us about the will o' the wisp in its Hobgoblin guise:
He oft out of a bush doth bolt,
Of purpose to deceive us.
And leads us on and makes us stray
Long winter's nights out of the way,
And when we stick in mire and clay
Hob doth with laughter leave us.