The 18th century Scottish philosopher, David Hume, had both feet firmly on the ground. He was a soi-disant savant on human nature and a particular expert on mankind's credulity. He did not believe, at all at all, in miracles.
"We soon learn," he wrote about such happenings, "that there is nothing supernatural in the case, but that all proceeds from the usual propensity of mankind towards the marvellous; and that though this inclination may at intervals receive a check from sense and learning, it can never be thoroughly extirpated from our human nature."
The mystery of the disappearing cumulus, perhaps, might be a case in point.
The scam was perpetrated many years ago by a man who declared that he could dissolve the clouds by merely staring at them. And it seemed he could! On the days appointed for his demonstrations he would point out a small cumulus cloud, fix his gaze firmly on it and - lo and behold! - after a short period it would be completely gone.
The feat, however, is not as difficult as one might think. On a relatively clear spring day the sky is often populated with lots of tufty, woolly, cumulus clouds. It might seem that this panorama overhead is just a moving backdrop, fixed in shape and drifting with the wind. But if you observe the scene more closely you will notice that each element of it is ephemeral; the pattern changes as the individual clouds develop and decay.
Cumulus clouds are isolated towers of tiny drops of water, and form when a volume of air near the ground has been warmed so that it acquires a buoyancy relative to the air surrounding it; these warm bubbles of air drift upwards, the rising air cools as it ascends, and eventually reaches the dew-point - the temperature at which condensation takes place. The result is a cumulus.
Now if you look closely at a cumulus cloud you will notice that while at first glance the cloud may appear to be fixed in shape, it is in fact constantly changing.
In a developing cumulus the cloud bubbles upwards, as the updraft surges higher and higher. But each cloud has a relatively short life-span - perhaps of half an hour or so - and the cloud dissipates as the vertical currents which have caused it die away.
A trained eye can spot a decaying cumulus with ease - and it would have been to such a cloud that our would-be Mesmer directed the attention of his audience; with a little luck it would have disappeared before their patience had run out.