It is generally believed that the Loch Ness Monster and the Yeti are, alas, mostly fictional; despite that, occasional eyewitness reports do surface from time to time.
Less suspect are the Northern Lights. A far greater number of people have experienced them; there is considerable hard evidence of their existence. And yet they remain somewhat unpredictable. Although it is possible to give a degree of probability when they might occur, it is by no means possible to guarantee their appearance.
Another such phenomenon is that beautiful cadence that occurs when rider and bike are in singular, joyous harmony; pedalling is perfect, effortless. The bike flies along, eating up the road with smooth, easy leg movements. At that moment, the headwind is transformed into a gentle, sweet breeze that thrills the face, the air is at once warm and cooling, the day is filled with life. Breathing becomes lighter, more like laughing, and the smile stretches wider.
This is a delicious, glorious sensation. And it is also fleeting.
No matter how long the cadence lasts - and sometimes it may be for miles - eventually, something changes. There is a shift in one of the many factors that influence the moment: the elusive, precise combination of gearing, fitness, energy levels, weather, road surface, terrain; all of the conditions that have come together to create the beautiful cadence go their separate ways.
And once more the bike ride is only that: joyful in its own way, but not without effort. The breeze becomes headwind once more; the gearing is slightly too hard (or slightly too easy, since you're spinning not grinding), the road lumpy and gritty where previously it was glossy and flat.
When might the moment return? It is, like the aurora, difficult to predict with confidence. Some days, on the same stretch of road, in the same gear, at the precise moment that the beautiful cadence visited, it's simply absent. At other moments it visits without warning, in unexpected places, at strange times; when the legs are tired and the hills long and the weather miserable, suddenly everything coincides for a few shining pedal strokes and the heart soars with the happy joy of cycling.
Before it vanishes again, slips away beneath the wheels, once more unknowable.
But that's what we're all pursuing, once we've experienced it, every one of us: cycling's beautiful cadence, the feeling of flying.