It is true that they also lay equally convenient to any one of the lesser havens upon the Sussex, the Kent, the Essex, or the Norfolk coasts, but the ample space afforded by the river, the opportunity for a crowd of shipping, coupled with opportunities for inland trading, were to be found nowhere else in the south and east of the island as they were to be found up London River. [Illustration: GRAVESEND] In this connection the use of the tide should be noted. By a peculiarity which has not been without its effect upon the history of the river the flood carries a man past the Kentish coast long past high water and indeed until a moment very close to that in which _another_ tide, that from the North Sea, carries its sweep of water up the river. Thus, though the tide reach high water shortly after noon in Dover Harbour, a man outside will carry the stream with him all up past the Kentish coast until close upon five o’clock. He has but to get round Longnose and he finds this other tide serving him--a tide that has been making from three o’clock or thereabouts, coming in from the North Sea, and that will carry him right up river as long as the wind or daylight permits him to follow it; a tide that does not reach its height at Gravesend until eight that evening, or in the Pool of London until nine. In other words, the River of London afforded to the vessel coming in from the south a double tide which, clever picked up, gave a continuous voyage from the Channel right down into the sheltered water inland. At this point it is interesting to consider why the principal harbours of the Middle Ages, in the north and west of Europe, at least, and upon tidal seas, so constantly developed not upon the coast itself, but at some distance up a stream or creek and inland. Consider the examples: Havre comes late in the development of the Seine--the original port is Harfleur; Bristol stands up a tortuous and narrow channel well inland, and not upon the estuary of the Severn at all; Liverpool had its first nucleus four miles from the open sea; Preston quite twelve; Chester more than twenty, of which the last five or six were a narrow river above the estuary; Nantes is another striking example--something like one day’s sail from the sea; Antwerp, Rotterdam--Bruges itself lying those few miles inland upon its canal--follow the general rule; and so does Norwich, and so does Colchester, and so does Boston, and so does Glasgow; Bordeaux tells the same story, and so does the Royal Harbour of Montreuil. And in general, while a great number of smaller ports rose upon the very coast of the Atlantic, the North Sea, or the Channel, there seemed to be, until modern times at least, a tendency for the main depôts of sea-borne commerce to lie thus inland. Why was it?