164

HISTORY OF

TRACT 6.

with a regular and uniform pressure upon it, would be ofabout 9000 tons. But when tbe effects of the vibration, whichmust necessarily take place in an arch of this magnitude, aretaken into consideration, the lateral pressure, or rathervibrating push, will far exceed that quantity ; and for thiseffort, as has been before noticed, provision must be made inthe strength of the abutments: and though the thickness ofthese in the design, namely 85 feet, seems to be great, yet Iam inclined to think it would be found too small, especiallyat the south end of the bridge, where I am informed theground is very bad, being moorlog and soft mud to a con-siderable depth. Indeed I should fear that something of thekind of what happened at Staines would be likely to take placehere, namely, the whole mass of masonry be forced back ho-rizontally, by the great lateral push of the arch, in spite ofevery precaution that could be taken to prevent it. But wemust observe, as we have before done in answer to theQueriesin the Report of the Committee of the House of Commons,that thefoundations of the abutments should belaid incliningtowards the centre of the circle to which the arch is drawn,as a more likely mode of preventing them from sliding out-wards, than if laid horizontally : but even with this precau-tion, if the substratum be moorlog or soft mud, it will be likelyto give way ; and if this ever take place, the abutment andarch must follow it.

The following is a rough sketch, on a very small scale, onthe design, at least very elegant, which was given along withthe above project.