History of the Peloponnesian War

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.