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to prevent the stones shifting on their beds. The likeliest use for them is to give point-hold to the crowbars used in forcing along the upper stones to their positions in the process of building.

The earlier bridge must have had at least one of its bays to the eastward of the embedded pier, and if only one, then the abutment belonging to it must have had its position underneath where is now the Roman Wall, and the roadway must have occupied the site of the north wall of the castellum. It thus seems clear that neither the castellum nor that portion of the Wall could have been coeval with the bridge.

A suggestion having been made that possibly the Wall had been lengthened when the later bridge was built so as to bring it forward to the bridge, Mr. Clayton gave permission to have the face of the Wall opened out eastwards, with the result that to a distance of sixty feet back from its junction with the castellum there is no break in the masonry, and the character of it is similar throughout, and very much like the exposed face on Limestone bank, the face stones running from about fourteen inches to nineteen inches in length, and from nine to twelve inches in depth. Writing in his Wallet Book, Dr. Bruce says of his portion of the Wall: It terminates in a square building or castellum formed of stones of the same character as those used in the Wall.' So far from this being the case the stones forming the Wall to the east of the bridge are larger, longer, and rougher than those in the castellum, these being nearly square on the face and very much smaller, and there is no true bonding between the two, the castellum having apparently been built on to the Wall end at a later time.

6

Mr. Clayton2 says: There is an apartment twenty-four feet by twenty-three feet six inches under the platform of approach.' This in the hands of Dr. Bruce becomes a castellum, and as the walls are well faced all round it could never have been designed for an underground chamber.

Owing to the dribbling away of material from under the abutment, the central portion, especially towards the face, and the castellum, have subsided considerably, but the longitudinal iron bonding of the face stones has held them so well together that no set or crack is perceptible in the masonry. Dr. Bruce thought that this depression of the centre portion was by design and deemed it an element of strength, but I scarcely think that any engineer would coincide in such opinion. 2 Arch. Ael. vol. vi. (N.S.) p. 82.

VOL. XVI.

Q Q

The peculiar splaying back of the face courses in the northern wingwall seems to be a scientific idea for accommodating the face-line to the different rates of flow in the river, i.e. giving a larger area to the more rapid surface water than was required for the comparatively sluggish current nearer the bed of the river, and it seems strange that engineers who could act upon such scientific lines should have made their piers flat-ended on the down-stream side, thereby incurring the danger of having the material eaten away from their foundations and the stones displaced by the regurgitative action of the water, and this too, with the evidence of the earlier piers before their eyes. It was this action of the water which rendered the lengthening of the south wingwall necessary, and in doing which they further endeavoured to throw the current away from the wall-face by placing the lower courses angle-way to the line of the work. This addition to the wingwall had been built chiefly with stone got from the earlier bridge remains, as is evident from many of the holes for the dove-tailed cramps remaining in positions which, in their new places, are of no use whatever.

It is a question whether in building their large ashlar work the Romans used mortar in the joints, or built it dry as was the custom in Rome under the Republic. In their smaller stone work such as the Wall with its camps, etc., they did use mortar, if indeed it may be so termed. Here and there patches of mortar may be found in a wellset condition, but, generally speaking, in the North of England it had been of a very poor character, the face of the stones merely ipped by a pointing of better mortar, and the hearting filled in with a mixture of badly slaked lime in clot, and soil instead of sand as a matrix, a material more calculated to disrupt than to cement the stones together; for, as the lime became hydrated, it would swell and tend to rend the work asunder. It would appear as if the designers, accustomed to the pozzolanas of Southern Italy and the limes of Tivoli, had looked slightingly on the comparatively inferior limes of the district and had not placed much reliance in their binding power, preferring, in their more important works, to trust rather to the more costly bonding of iron run in with lead.

From the evidence existing, pointing as it does to the later construction of the Wall, it may be taken that the earlier bridge was

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antecedent to its erection, and the question arises what office was this earlier bridge designed to fulfil? It seems to be generally agreed that previous to the building of the Wall, Agricola had constructed a chain of forts across this isthmus, and as these would almost certainly be connected by a line of road, it is possible that this bridge might have been built in connection with such road, though as subsequently noted in this paper, I think the probability is against it. Then as to the date of erection of the later bridge, it seems unlikely that during the short period between Agricola and Hadrian (about forty years) the river had time to alter its course a distance of sixty feet from its former line, as it has taken 1,700 years since the departure of the Romans to perform an equal distance in the same direction. When Severus returned from his northern campaign, about 130 years after the time of Agricola, would appear to be a much more likely time for the river so to have changed its course and for the later bridge to have been built.

Amongst the débris of the bridge abutment there are certain peculiar shaped stones which have evidently been designed for some special use. One of them is a monolithic pillar, nine feet one inch in length, having a rectangular base, two feet two inches by one foot eleven inches, for a height of two feet two inches from the bottom; above this the angles are rounded off, until at the top it assumes the circular form with a diameter of one foot seven inches. The shaft of the column is six feet six and a half inches long and concentrically on its upper end, there is a curved conical boss, four and a half inches deep, with a scarcement all round it of five inches on the pillar top. On the longer face of the base the stone has been cut away to a depth of five inches, so as to leave projecting a face moulding, and as the shape of this moulding is similar to that upon other stones which have apparently formed an ornamental string course along the face of the abutment, the original position of the pillar stone is thereby determined as having been on the face of the abutment and in line with the string course. As another evidence of the position of the pillar stone, there remains one of the stones which had formed the parapet hollowed out to fit up against it.

There are also portions of a similar column which had been broken up. The upper end of it is now on the abutment amongst the ruins,