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At Station 3. The average reading for high water was 6.30, for low water 10.74, giving a mean range of 4.44 feet. The spring tides during the lunation, rose above the mean high water level by 0.62 foot, and fell below the mean low water level by 0.52 foot, giving a range for springs of 5.58 feet. The neap tides did not rise to the mean high water level by 0.34 foot, nor fall to the mean low water level by 0.76 foot, giving a range for neaps of 3.34 feet. The rising of the water at No. 3, in relation to the rising at No. 1, was observed to vary from rare coincidence therewith to thirty minutes thereafter. At three-quarters flood, the tide at No. 3 was from ten to twenty-five minutes behind the tide at No. 1, lagging most at springs and least at neaps. High water at No. 3 was not attained, till after it had been reached at No. 1, by times varying from ten to thirty minutes.
Assuming that the observation at No. 1 indicate the normal tidal condition for this region we may, by comparing the observations at Nos. 2 and 3 therewith, discover in what directions and to what extent this condition has been altered at these points.
First-As to the rate of propagation: The stations being in such proximity, propagation between them should be almost instantaneous. It does not appear from the observations that propagation between 1 and 2 is to any great extent interfered with. But between 2 and 3 the propagation is so far obstructed that at times it requires thirty minutes for the effect of the rising tide apparent at No. 1 to be felt at No. 3. An idea of the extent of the retardation at ordinary spring tides may be had by referring to the appended diagram, which is a plotting of observations on an actual tide. By following up the vertical line corresponding to any hour, one may read off, at its intersection with the curves, the heights at the several stations at that time. Or by followlowing the horizontal line corresponding to any foot, one may read at its intersection with the curves, the times which elapsed between the attainment of this height at the various stations. This diagram shows the tide at No. 2 lagging generally 6 minutes behind that at No. 1; the tide at No. 3 lagging 25 minutes behind that at No. 1.
Second-As to the low-water plane: The mean low-water reading at No. 1 was 10.84, at No. 2 10.78, at No. 3 10.74, giving a rise in the mean plane, from 1 to 2 of 4 of an inch, and from 2 to 3 of ļ an inch, or a total rise from 1 to 3 of 14 inches. At neaps the rise in the low-water plane from 1 to 2 is of an inch ; from 2 to 3 the plane is level. At springs the rise in the plane from1 to 2 is 11 inches, from 2 to 3, 13 inches, giving a total rise from 1 to 3 of 21 inches.
Third—As to the range of the tide: The mean range is of an inch less at No. 2 than at No. 1, and i of an inch less at No. 3 than at No. 2, giving a total loss at No. 3 of 14 inches. For neap tides the range is z of an inch less at No. 2 than at No. 1, and } an inch less at No. 3 than at No. 2, or a total loss at No. 3 of 13 inches.
For springs the range is 1} inches less at No. 2 than at No. 1, and 14 inches less at No. 3 than at No. 2, giving a total loss of 3 inches in the range of spring tides for the reach of Seekonk river above Washington bridge. In other words, at spring tides, 191 million cubic feet of water, which, in the normal condition of the tides would daily enter and withdraw from the Seekonk, is in some way prevented from so doing.
The two most important effects on the Providence navigation of these changes in the orderly flow of the tides are, the creation of violent currents and the loss of scour.
The first of these effects is apparent to any observer standing on the bridges. A clear idea of its cause may be had by examining the appended diagrams of the flow and ebb of an ordinary spring tide. These diagrams show approximately the forms assumed by the surface of the water during different stages of the tide, and are made up from the readings of the tide, the curves of which are also shown. The forms are but approximate, since in all probability the incline which appears in them belongs altogether to a short space extending on either side of the bridges,—the lines from 1 and 3 being horizontal to quite near the bridges. On these diagrams a graduated scale is shown at each station, the feet being marked with Roman numerals; at the right the times appear in Arabic. They show, for example, that at 6.40, on the flood tide the reading at No. 1 was 6.8, while owing to retardation, the reading at No. 2 was 7.13, and at No. 3, 7.65, giving during the latter half of the flood a fall on the surface of the water between