REPORT ON HYDRAULICS. PART 11. 4Sd 



the difference of level was found to be occasionally as much as 

 fourteen inches between the high water below and above bridge, 

 and five feet seven inches between low-water mark above and 

 below bridge, depending of course on the state of the freshes and 

 tides. The bridge was considered to act like a pound-lock, and, 

 by penning up the water, to tranquillize the motion of the cur- 

 rent, and deepen the navigation above. In consequence, how- 

 ever, of the danger and inconvenience arising from both the im- 

 peded navigation through the bridge and the floods, Mr. George 

 Dance was instructed by the Corporation of London, in the year 

 1746, to draw up a series of queries, which were addressed to the 

 Royal Society. 



The result was a Report from the Society requesting certain 

 information relative to the tides, which however did not elicit 

 anything positive upon the subject until the year 1754, when the 

 erection of Blackfriars Bridge was contemplated. The opinions 

 of Mr. Robertson, as detailed in Dr. Button's Mathematical 

 Tracts, were given on the unfounded supposition that the pro- 

 posed bridge was to be built with piers and starlings like London 

 Bridge, and to produce a similar obstruction. The enlarge- 

 ment of the water-way in the year 1759, by lowering the 

 surface of the water several inches, caused a diminution both 

 in the depth of the water, and in the power of the water- works. 

 The area of the water-way was again contracted, and the river 

 restored to its former state, on the supposition that the naviga- 

 tion would have been otherwise injured, and the low lands over- 

 flowed. And when the question of rebuilding the bridge came 

 to be agitated, it was argued, That the old bridge acted as a bar 

 to check the velocity of the river both ways ; — that an increased 

 velocity in the river would impede rather than accelerate the 

 navigation, as wherries and small craft could not stem the 

 current; — that the bed of the river would be laid dry during 

 the ebb tide; — and, lastly, that the upper part of the river 

 would be choked with mud, and all the low grounds on either 

 side of the river would revert to marshes and be rendered un- 

 inhabitable. 



On the other hand it was contended. That the tides would not 

 rise more than a few inches higher than formerly, or fall lower 

 than three feet; — that the old bridge not only acted as a 

 dam to check the flux and reflux of the tides, but tended 

 to pen back the land-waters, and to cause floods above ; and 

 that the proof of the bridge causing such an efi^ect was 

 the greater prevalence of floods before the enlargement of the 

 waterway of the old bridge in the year 1759, than afterwards ; — 

 that the deci'ease in the velocity of the river tended to assist 



