History of the Peloponnesian War

Toward the close of the same winter, when spring was near at hand, Brasidas made an attempt on Potidaea. He came up by night and placed a ladder against the wall, up to this point escaping detection; for the ladder was planted precisely at the interval of time after the bell had been carried by and before the patrol who passed it on had come back.[*](It appears that the bell was passed from one sentinel to the next. Another, and probably more common, way of testing the watchfulness of the sentinels was to have a patrol with a bell make the round, each sentinel having to answer the signal.) The guards, however, discovered it immediately, before an ascent could be made, and Brasidas made haste to lead his army back again, not waiting for day to come.

So ended the winter and with it the ninth year of this war of which Thucydides wrote the history.