Alberuni has made a clear distinction between a gulf and an estuary. An estuary, according to him, is an inlet or an arm of the sea, specially the wide mouth of a river, where the tide meets the current. His definition of an estuary is very near to our present definition. He admits that an estuary does have a similarity with a gulf but it is not formed like the latter by the penetration of the ocean into the continent. He remarks that an estuary is 'formed by an expanse of flowing water, which is changed into standing water and is connected with the ocean'.⁷⁵ He warns against the dangers of navigation in an estuary because the sweet water in an estuary being light is not capable of bearing upon it very heavy bodies. The explanation given by Alberuni may appear to us ridiculous but we do know...