THE Battle of LePanto took place in the Greek waters of the Gulf of Corinth in 1571. Forty thousand soldiers and sailors lost their lives, and almost as many were wounded when the Catholic forces' of the Holy League, led by the charismatic Don John of Austria, defeated the Turks in what was to be the last great naval engagement between oared galleys.
The encounter was celebrated by O.K. Chesterton in what must be one of the jauntiest poems of all time
Strong gongs groaning as the guns boom far,
Don John of Austria is going to the war.
Torch light crimson on the copper kettledrums,
Then the tuckets, then the trumpets, then the can non and he comes
Don John laughing in the brave beard curled.
A few of the contributors to this cacophony would have been aware that the sound of cannon fire behaves in a rather peculiar way. Most sounds, as we know, become inaudible at a relatively short distance from their place of origin, as the sound waves attenuate, or weaken, and eventually die away.
But centuries ago it had been noticed that the sound of gunfire was often heard, not only in the immediate vicinity, but also in an outer ring 60 miles or more from the source, with a zone of silence in between. It was also noticed the sound appeared to take an unusually long time to reach this outer region.
Not until the early years of this century was the reason for this anomaly deduced. Listeners in the inner zone hear sound waves which have travelled directly towards them through the lower atmosphere, but the existence of the outer zone can only be explained by the hypothesis that waves originally moving upwards from the source are subsequently redirected downwards again to reach the earth a great distance away.
And the only reason sound waves should so dramatically change their direction of propagation would be the existence of a region of relatively high temperature high above the Earth.
Early meteorologists put this hypothesis to good use. During the 1920s they took accurate measurements of the dimensions of the "silent" and "noisy" zones associated with prearranged loud explosions, and also measured the length of time taken by the sound to reach a selection of different sites.
From these data, the researchers calculated that a warm layer existed in the atmosphere between 20 and 40 miles above us, with temperatures not too different from those we experience here on the Earth's surface. These conjectures have been amply confirmed in the intervening years.