upon a later page when I speak of urban life upon the Thames.  The nomadic element in the life of the early Middle Ages; the smallness of the space allotted for sleeping; the large amount of time spent out of doors; the great proportion of collegiate institutions, not only monastic but military; the life in common which spread as a habit to so many parts of society beyond the monastic; the large families which (from genealogy) we can trust to be as much a character of the early Middle Ages as they, were not the character of the later Middle Ages, the crowd of semi-servile dependants which would be discovered in any large house—­all these make us perfectly safe in multiplying by at least ten the number of households counted in the Survey if we would get at the population of those households, and it must be remembered that the houses counted, even in those parts of England which were fairly thoroughly surveyed, can only represent a minimum number, whatever was the method of counting.  The lists may in some instances include every single household in a place, though from what we know of the diversity of local custom this is unlikely.  In most places it is far more likely that the list covered but some portion that by custom owed a public tax, and this is especially true of the towns.

After Dorchester, which was the first of the fortresses of the Thames, so far as we have any knowledge, and after Oxford, which came next, and appears to have been founded since the beginning of recorded history in these islands, there remain to be considered the other strongholds which held the line of the valley.

It would be easy to multiply these if one were to consider all fortifications whatsoever connected with the general strategic line formed by the Thames, but such a catalogue would exceed the boundaries set to this book.  It is proposed to consider only those which were strictly connected with the passage of the stream, and of such there are but three besides Dorchester and Oxford, for that at Cricklade is doubtful, and in any case determines a passage which could be always outflanked upon either side, while the great fortress of the Tower, lying as it does upon the estuarial Thames below bridges, does directly protect a highway.

These three strongholds directly connected with the inland river are Wallingford, Reading and Windsor, and of the three Wallingford and Windsor were more directly military:  the last, Reading, appears to have been but an adjunct to a large and civil population; the fourfold quality of Reading in the history of the Thames, as a civil settlement, as a religious centre, as a stronghold, and as one of the very few examples of modern industrial development in the valley, will be considered later.  We will take each of the three strongholds in their order down stream.

What determined the importance of Wallingford is not easy to fix nowadays.  The explanation more usually given to the great part which this crossing of the Thames played in the early history of Britain is the double one that it was the lowest continuously practicable ford over the river, and that it held the passage of the great road going from London to the west.