Every now and then in the learned scientific journals, you read that someone has produced a new theory to explain ball lightning. Then you hear no more about it, until you read about another theory six months later.
The phenomenon remains one of the great unsolved mysteries of meteorology - a riddle wrapped in a mystery inside an enigma, encapsulated in a little glowing sphere.
Ball lightning is very rare, but it has been seen sufficiently often by reliable witnesses not to be dismissed as hoax or mere imagination. The "fireballs", as they are sometimes called, may be white, red or orange in colour, and range in size from beans to basketballs.
The apparition is often accompanied by a hissing sound and a distinctive smell; it typically drifts along some feet above the ground, as if wafted in a current of moving air; then after several seconds it disappears as quickly as it came.
It has a great affinity for buildings, which it enters through open doors and windows or down chimneys. This strange fireball seems to be relatively harmless - "lambent but innocuous", as a certain cleric once described the scintillations of a colleague's wit. It may occasionally cause a little damage, but rarely of a serious nature, and people coming into contact with it describe the sensation as like receiving a glancing - albeit somewhat painful - blow.
Although it has been seen hundreds of times, scientists are still nonplussed about its origins. Most occurrences take place in thundery conditions, when ordinary lightning has been seen as well, but the appearance of ball lightning is so unpredictable that systematic observations of the phenomenon have so far proved impossible. It has been suggested it may be some form of "brush discharge" of static electricity in the atmosphere, or a vortex of air containing a dense concentration of inexplicably luminous gases.
The most promising studies have been carried out in the laboratory. In 1990, for instance, two Japanese scientists, Ohtsuki and Ofuroton, succeeded in producing a promising example of something like ball lightning by passing a powerful electrical discharge through an atmosphere mildly impregnated with an inflammable gas.
But is it really ball lightning? No one knows. Some scientists refuse to believe that it exists at all, asserting that it is merely an optical illusion caused by an after-image on the retina of the eye immediately following a lightning flash seen by the observer.
They place it in the same category as Hamlet snr's ghost: "Horatio says 'tis but our fantasy, and will not let belief take hold of him."