History of the Peloponnesian War

The same winter the Plataeans, (for they were still besieged by the Peloponnesians and Boeotians,) when distressed by the failure of their provisions, and when there was no hope of aid from Attica, and no other means of safety presented itself, both themselves and the Athenians who were besieged with them formed a design, in which they were at first unanimous, for all to sally forth and pass the walls of the enemy, if they could force their way over them; the attempt having been suggested to them by Theaenetus son of Tolmidas, a soothsayer, and Eupomidas son of Daïmachus, who was also one of their generals.

Subsequently half of them shrank from it, thinking it a great risk; but about two hundred and twenty voluntarily persevered in the attempt, [which they effected] in the following manner.

They made ladders to suit the height of the enemy's wall, measuring by the layers of bricks, where the wall looking toward them happened to be imperfectly whitewashed. Now many counted the layers at the same time; and though some would naturally miss the correct calculation, the majority would attain it; especially as they counted them many times over, and were also at no great distance, but the wall was easily observed by them as far as they wished.

In this way they ascertained the proper length of the ladders, guessing the measure from the thickness of the bricks.