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THE PRINCIPLES OF BRIDGES.

TRACT 4.

disposed of in the best manner that its situation willadmit,

“ 6. On account of the water being confined between thebanks of the river, the tide must flow up higher, in propor-tion as the river becomes narrower, till the fixed quantity isreceived. But then it must be observed, that when the tideacts against the stream of a river, the tide up that river be-comes progressively stronger and stronger, for a time, ac-cording as the velocity of the natural stream is checked; andin this manner the river waters themselves by degrees obtaina contrary direction, and run up with the tide, and so maybe considered as waters coming in with the tide of flood,and part of the fixed quantity which that river is to receive.

“ 7. The return of the tide, or the time of ebbing, is notevery where performed in the same time as it took to flowin. For, in the ebb tide there is to be discharged, not onlythe waters which were brought in by the tide, but also all theriver water which has been retarded by it.

“ 8. Whatever obstacles are laid in the way of the tide,across any channel, the utmost rise, or the high-water mark,at different times, will be respectively the same : because thewater will continue to rise till the fixed quantity of tide isdisposed of, and no longer. And, in like manner, the low-water mark will not be affected by such obstacles. Indeed,between the limits of high and low-water marks, the waterwill be raised higher against those obstacles, both in the floodand ebb tides, than they would be in those places, were theobstacles removed. For, as the velocity of the current must,on both sides of the obstacle, be equal, in order for one partof the water to run away, as fast as the successive ones fol-low ; therefore the waters must rise on that side of the ob-stacle which they run against, till they be so high, that bytheir fall they acquire a velocity sufficient to carry them off,as fast as they arise at the obstacle.

“ These principles being premised, the solution to thequestion proposed naturally follows. And in order to this,let us for the present suppose, that between London and