« السابقةمتابعة »
NORTH TYNE NEAR CHOLLERFORD.
previous termination. The river at this point has altered its course from time to time in a westerly direction, so that now the eastern abutment and a portion of the first pier are deeply buried in its bank, and the western abutment is in the bed of the stream.
But this action of the river had been in operation previous to the time when this bridge was built, for, embedded in its eastern abutment is a water pier of an earlier bridge which must have had at least one bay or opening to the east of it, so that between the times when the two bridges were built the river had altered its course to that extent westward.
The roadway along the earlier bridge had been much less in width than that along the later one, its pier faces being only nine feet four inches long with a width of ten feet four inches. The earlier bridge had starlings both up and down stream diverging from the pier faces at an angle of forty-five degrees. It is deserving of note, and unique in my knowledge of bridges, that these piers should be less in length than their breadth across, but as the bridge had doubtless a timber platform, the beams supporting it would require to be cantilevered by others underneath them to give rigidity, and the breadth of the pier would be necessary to afford a sufficient length of base for them. The lowness of the roadway would render angle strutting to the piers useless from the certainty of their being carried away during floods.
The spans of this bridge must have been abnormally long in comparison with the width of the roadway which could only have been about eight feet six inches, for a length from pier to pier of forty-one feet two inches or nearly in the proportion of five to one. If, however, the roadway platform had been projected beyond the longitudinal bearing timbers at each side, a greater width would be obtained.
The builders of the later bridge had taken full advantage of what remained of the former one, for not only had they inbuilt the water pier in their abutment, but had adopted and enlarged the other piers also, by adding a width of five feet eight inches to their western sides, and lengthening them southwards. This is apparent on applying a tracing of the embedded pier (the dimensions of which can be exactly defined) to the plan of the second pier, as given on the accompanying drawing, where the different character of the masonry clearly defines the earlier and more recent work.
The western abutment of the earlier bridge would most likely occupy the site of the third river pier of the later one, and if one bay be allowed to the east of the embedded pier the number of the openings in the two bridges would be equal, only the widths of these openings would be five feet eight inches more in the earlier than in the later one.
It seems pretty clear that the skill of the bridge builders had not improved as time went on, for the masonry of the later bridge, though substantial in character and composed of large ashlar stone throughout, is not nearly so well bonded by snecking and breaking joint with the stones as in the earlier one, though in the mechanical appliances of setting the stones they seem to have advanced, for the lewis had been adopted in place of the hand setting of the former work.
The stones in both bridges have been elaborately fastened together by iron cramps and ties run in with lead, but here again the earlier men seemed to be in advance of the later ones, for instead of the long iron-face straps with T-headed branches running in a sort of haphazard manner into the work, and the few dog cramps here and there of the later work, there are systematic double dove-tailed cramps of good form, neatly let into the stones.
Dr. Bruce inclines to think that the facing stones of the abutment of the later bridge might have been an addition by Severus to what he terms Hadrian's work, but I think that an examination of the plan will show that where these impinge on the embedded pier it could not have been so, and that this casing is an initial part of the second structure, and coeval with the added work of the piers where the same long iron clamps have been used.
The work generally of the later bridge is of a ruder character than that of the earlier one, and there are many make-shifts apparent, indicating that the workmen had not such intelligent overlookers. This is apparent in the way many of the upper faces of the stones had to be dressed down after being set to admit of the proper bedding of those above. And there is a piece of very unconstructive work where the southern wingwall had been lengthened.
In the beds of the stones forming the earlier pier there are at uncertain intervals wedge-shaped holes, the use of which is not clear. They could not have been used in lifting, and are not holes for joggles