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In 1878 the harbor commissioners of Rhode Island had some levels run along the river, using as a datum the reference to mean low water on the west abutment of India Point Railroad Bridge, which was established there in 1874 by the United States Coast Survey, while making a survey of Providence Harbor. These levels showed a difference of elevation of mean low water between India Point Bridge and Butler Hospital Wharf of 1.35 feet.

This difference was too great for truth, and examinations were instituted to discover the error.

The following is a description of the results obtained in this connection, together with some observations on the question of the improvement of the river incident to it:

GAUGES ESTABLISHED AND LEVELED BETWEEN IN 1879.

At City Wharf, Providence.-This gauge (the same as described in report of this date on Providence River tides) is located near Point Street Bridge, but for all purposes of comparison it will be considered as being located 100 feet below India Point Railroad Bridge, observations having shown that there is no difference in either time or height between these two places. It is at India Point Bridge that the complications in the tides of the Pawtucket River arise. The gauge was placed at City Wharf for use in the Providence River observations, and was observed for the two purposes at the same time. Mean rise and fall of tide, 4.7 feet. Mean low water is referred to a bench-mark on top of west abutment of India Point Railroad Bridge. The highest point of the letter S, of the letters C. S. cut in the abutment, is 11.909 feet above mean low water.

At Red Bridge.-The gauge here is located about fifty feet above the bridge. It was observed for a lunar month. Mean rise and fall, 4.57 feet. Bench-mark northeast corner of bridge seat of west abutment of bridge is 17.132 feet above mean low water. The mean low-water at City Wharf, Providence as referred here by levels, is the same as that taken from the month's observations just referred to.

At Butler Hospital Wharf.—This gauge is on the wharf at the same place it was when taken in 1873. Observations were made for a lunar month; they gave a mean rise and fall of 4.84 feet, or 0.38 foot more than was recorded in 1873. The mean low water reference differed only 0.03 of a foot. The difference of elevation discovered by the harbor commissioners in their surveys in 1878 is accounted for by the fact that the wrong point was mistaken for the bench-mark. The bench-mark is the bottom step of the upper river face of the wing wall of bridge over road-way leading from the wharf to the hospital buildings; its elevation is 55.825 feet above mean low water of the observations of November, 1879, and 55.794 feet above that determined in 1873. Mean low-water at City Wharf, Providence, is 0.07 foot below that established here in 1879.

At Dunnell's Wharf.-This gauge is located on the wharf about 1 mile below the dam, which is just above the sudden rise of the bed of the river, which makes it the limiting point of the tidal influence, and where the river changes its name to the Blackstone. The observations were made here so that the volume of fresh water coming in from the Black stone would not affect them so

much as it would had they been taken nearer the head of the river. The mean rise and fall of tide here is 4.82 feet. Mean low water is referred to a benchmark which is a crow-foot mark on top of most southerly pile in Dunnell's Wharf; it is 6.848 feet above mean low water; also to a bench-mark which is a vertical and horizontal crow-foot mark on thin flat stone of wall of wharf, second stone from top and 11 feet from southwest corner of Dunnell's Wharf It is 6.821 feet above mean low water. Mean low water at Providence is 0.22 foot below that established here in November, 1879.

The following is a table of observations made at high and low water at each of the above-mentioned places; the datum line is the line of mean low water, Providence.

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A study of this table shows the low tide to be generally the same as regards height at City Wharf, Providence, and Red Bridge, while at Butler Hospital Dock it is a little above, and at Dunnell's Wharf more above.

The high tides at Red Bridge are below those of City Wharf, Providence; at Butler Wharf they are above City Wharf, and still farther above at Dunnell's Wharf.

Dunnell's Wharf.

DURATION OF STAND AT LOW AND HIGH WATER.

During seven days, November 17 to 23, inclusive, careful observations of the time of stand were made at each gauge; the results are shown in the following tables:

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The next two tables give the number of minutes elapsing between the time that low-water commenced to rise and high-water to fall, at India Point, Red Bridge, and Dunnell's Wharf.

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The direction and force of the wind on these days was as follows; 5-gale, 0-calm:

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The foregoing tables show the tide falling at Providence 27 minutes (mean of

7 days) before it is affected at Dunnell's Wharf.

They also show the low-water to have commenced to rise 52.2 minutes earlier at Providence than at Dunnell's Wharf. The Coast Survey table of constants gives as the difference of time that the tidal-wave reaches Newport and Providence as 29 minutes, a distance of about 29 miles. If we suppose this rate to be the natural one, then the difference between Providence and Dunnell's Wharf would be but 5 minutes, or 23 minutes less than is actually the case on the falling tide, and 47 minutes less than the rising tide, but this is supposing the Seekonk River to be an unobstructed estuary of Narragansett Bay and the ocean. Very far from being unobstructed, this river at its mouth is naturally narrow and is artificially obstructed to a large extent by the foundations of the bridges. In order to ascertain, as nearly as it is practicable to do so, how much of the retardation of the tide is due to natural and how much to artificial causes, I have taken the time that elapsed at high-water, between the time it commenced to fall at Dunnell's Wharf and India Point, as indicating approximately the rate of propagation uninfluenced by the artificial obstructions. At high-water the volume of water in this basin is increased about 400 per cent., while the obstructions at the bridges are no greater than at low-water; their influence, therefore, should be but one-fourth as much at highwater, and consequently the retardation of the first of the falling tide should be proportionately less, which the observations show to be the case.

The duration of the rise at Providence (mean) is given by the Coast Survey as 7 hours 10 minutes, and of the fall 5 hours and 15 minutes, or 1 hour 55 minutes less for the fall than the rise. If the full ebb-tide has an acceleration over the flood of 1 hour and 15 minutes, the difference in time which is required for the tide to go from Providence to Pawtucket in the same proportion would be 10.1 minutes, so that if the observations show the falling tide to be 27.8 minutes in getting between the two places, 27.8 plus 10.1 should give the time of the rising tide in making the same distance. The observed time of passage of flood-tide is, however, 52.2 minutes, or 14.3 minutes in excess (38 per cent.), which may be taken as an approximate measure of the effect of the obstructions at the bridges on the rate of tidal propagation.

The following map shows the tidal basin of the river with the location of the observed gauges indicated. On the same sheet is platted a diagram of tides on which is shown the extreme and mean high and low waters referred to the datum of mean low-water, Providence.

The effect of the obstructions at the bridges may be readily seen from this diagram. In the obstructed area between Red Bridge gauge and India Point the mean low-water line is level; the high-water line of November 21 descends from India Point to Red Bridge 3 inches, and then rises to Dunnell's Wharf 6 inches; the mean high-water line descends 3 inches to Red Bridge, and then rises 6 inches to Butler Hospital and 2 inches more to Dunnell's Wharf; the extreme high-water of November 15 is level to Red Bridge, and then rises to Dunnell's Wharf 9 inches. We thus have in the cases of mean high-water and high-water of November 21 a slope extending up river in the obstructed area, while it is reversed and extends the other way or down river in that part above Red Bridge where it is open and free. In the case of extreme high-water the effect on the increased mass of water is not so apparent, and this line becomes level up to Red Bridge, and probably with a still greater body of water a uniform slope would exist throughout.

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