مِرْجَاسٌ

A stone which is tied to the end of a rope, and which is then let down into a well, and stirs up its black mud, after which the water is drawn forth, and thus the well is cleansed: (S, K: *) or a stone which is thrown into a well in order that one may know, by the sound thereof, its depth; or that one may know whether there be in it water or not: (IAar, K:) or, according to ISd, the name by which this is known is مِرْدَاسٌ. (TA.)