Plethrum

The Plethrum (Plethron) is a Greek measurement of 100 Greek feet in length (101 English feet or about 30 metres) as well as 10,000 sq.ft. in area. It and its plural, plethra, is used in four instances in Critias (115d, 116a, 116d and 118c).

It is interesting to note that in one case (118c) Plato found the dimensions of the 10,000 stade ditch surrounding the plain of Atlantis as very hard to believe but felt obliged to record it out of deference to the reputation of his source, Solon. The text states that this ditch was dug by hand to a depth of a plethrum and a stade in width.

The late Ulf Richter argued that Solon had been given dimensions in Egyptian khets (52.4 metres) but that he had noted them as Greek stades (185 metres)^(a).

He was forced to make this suggestion because he recognised that the dimensions for the plain of Atlantis as recorded by Plato must have been exaggerated. However, when it came to the ditch which surrounded the plain he was also contented to accept that the stade width of the ditch was excessive but that the plethrum depth was acceptable because the clay banks would have been continually subject to slippage! Feasible, but for me, not totally convincing.

>The most recent entrant into this particular debate is M.P. Courville ^[1960.10] who has proposed that Plato intended to use the plethrum, which is around 30 metres long or very roughly one-fifth the length of a stade. The problem arising from that idea is that Plato also uses the plethrum together with the stade in contexts where they both seem too large (Crit.116a & 118c)!<

^(a) CiteSeerX — Plato´s Atlantis was in a River Delta (archive.org)