Whitsuntide 335 years ago was a stressful time in London. The Anglo-Dutch Wars were in full swing, with the opposing fleets engaging with each other regularly.
One such naval skirmish, the so-called Four Bay Battle, took place in the early days of June, 1666, along the continental coast between Dunkirk and Ostende. Samuel Pepys, as always, recorded events methodically in his diary.
On June 2nd, he wrote: "I went to Greenwich with Captain Erwin, and into the park, and there we could hear the guns from the fleet most plainly." But the following day he heard about passengers aboard the yacht Catherine in the English Channel, "who saw the Dutch fleet on Thursday and ran from them; but from that hour to this hath not heard one gun nor any news of any fight".
The Governor of Dover Castle dismissed reports of gunfire heard in London as "only a mistake for thunder".
Pepys sums up the situation as he saw it in his entry for June 5th: "It is a miraculous thing that we all Friday, Saturday and yesterday, did hear everywhere the guns most plainly, and yet at Deal and Dover they did not hear one word of a fight, nor think they heard one gun. This, added to what I heard about Catherine, makes room for a great dispute in philosophy - how we should hear it and they not, the same wind that brought it to us being the same that should bring it to them: but so it is."
Pepys's account may well be the first written analysis of a now relatively well-known phenomenon. Most sounds, as we know, become inaudible at a relatively short distance from their place of origin. The sound waves attenuate, or weaken, as the oscillations of the tiny particles of air become less vigorous and eventually die away.
But the sound of gunfire is often heard, not only in the immediate vicinity, but also in an outer ring 60 miles or more away from the source, with a zone of silence in between.
The reason for this anomaly was deduced in the early years of this century. Listeners in the inner zone hear sound waves which have travelled directly towards them through the lower atmosphere. The existence of the outer zone is explained by the existence of a layer of relatively high temperature above the Earth. Sound waves originally moving upwards from the source are deflected from their straight-line path by successive layers of air of increasing temperature. They are ultimately re-directed downwards again to reach the Earth a great distance away, well beyond the intervening silent zone.