Make straight

Until the early nineteenth century, in the French Jura, when a forest was being divided up before felling, two men would stand a certain distance apart and call to one another. A third man would try to position himself in the middle by listening to the shouts and answering them. All three would then call out at regular intervals while a fourth man walked along the path of sound, marking the trees, and when that section of the forest was felled, the result was an astonishingly straight line. The same acoustic surveying technique was used by Persian road-builders, and it was this that gave the prophet Isaiah the image of a voice crying out in the wilderness, ‘Make straight in the desert a highway’.

from Graham Robb, The Ancient Paths