a great navigable river of England, composed chiefly of the river Isis and Thame; of which the Isis is much the largest, and runs the longest course, rising on the confines of Gloucestershire. At Lechlade it becomes navigable, from whence it continues its course north-east to Oxford, where it receives the Charwell; from Oxford it runs south-east to Abington, and so to Dorchester, where it receives the Thame, and continues its course south-east to Windsor, and thence runs east to London, and continues the same course to the sea, receiving the river Medway near the mouth of it. The Thame is but a small river, which rising near Tring in Hertfordshire, crosses the county of Bucks, and falls into the Isis at Dorchester.