You will see, then, by looking at any map, that Scotland is separated from England by two rivers which flow from the interior of the country into the sea--one towards the east, and the other towards the west. The one on the east side is the Tweed. The Tweed forms the frontier between England and Scotland for a considerable distance, and is, therefore, often spoken of as the boundary between the two countries. Indeed, the phrase "beyond the Tweed" is often used in England to denote Scotland. In former times, when England and Scotland were independent kingdoms, incessant wars were carried on across this border, and incursions were made by the chieftains from each realm into the territories of the other, and castles were built on many commanding points to defend the ground. The ruins of many of these old castles still remain. On the western side of the island the boundary between England and Scotland is formed by a very wide river, or rather river's mouth, called Solway Frith. Between this Solway Frith and the Tweed, the boundary which separates the two countries runs along the summit of a range of hills. This range of hills thus forms a sort of neck of high land, which prevents the Tweed and the Solway Frith from cutting Scotland off from England altogether, and making a separate island of it. About seventy or eighty miles to the northward of the boundary the land is almost cut in two again by two other rivers, with broad mouths, which rise pretty near together in the interior of the country, and flow--one to the east and the other to the west--into the two seas. These rivers are the Forth and the Clyde. The Forth flows to the east, and has a very wide estuary,[B] as you will see by the map. The Clyde, on the other hand, flows to the west. Its estuary is long and crooked. [B] An estuary is a sort of bay, produced by the widening of a river at its mouth. Scotland is remarkable for the estuaries which are formed at the mouths of its rivers. They are called there _friths_. The Forth and the Clyde, with their estuaries, almost cut Scotland in two; and by means of them ships and steamers from all parts of England and from foreign ports are enabled to come into the very heart of the country. The two largest and most celebrated cities in Scotland are situated in the valleys of these rivers, the Forth and the Clyde. They are Edinburgh and Glasgow. Edinburgh is on the Forth, though situated at some little distance from its banks. Glasgow is on the Clyde. There is a railway extending across from Edinburgh to Glasgow, and also a canal, connecting the waters of the Forth with the Clyde. The region of these cities, and of the canal and railroad which connects them, is altogether the busiest, the most densely peopled, and the most important portion of Scotland; and this is the reason why Mr. George wished to come directly into it by water from Liverpool.