24
THE TIDAL COMPARTMENT OF A RIVER
abate or disappear. On the river Tsien-tang-Kiang1 bore effects exist in an unexampled degree. As the flood travels across the sand there is a difference of level of 19 feet at springs between the water on the outside of the bar and that in the mouth of the river, a distance of about 20 miles. The measured speed of this flood having a gradient of about one foot per mile is 14.6 miles per hour. The bore has a breadth of 1800 yards, and forms a cascade 8 to 12 feet high. It strikes the outlet of the river at an angle of 40 to 70 degrees, and its roar is audible 14 or 15 miles away, an hour and twenty minutes before arriving. The bore maintains its breadth, height, speed, and regular appearance for 12 to 15 miles above the mouth of the river. At the city of Hang-chau, 24 miles from the mouth, the tidal range drops from 19! feet to 6 feet.
   The ancient devices of the Chinese to protect the native river boats from the effects of the bore have been adopted for the last eight or nine hundred years and still persist. The river is embanked for a distance of about 30 miles, and the top of the sea-wall is 3 to 9 feet above high-water spring tides. The embankment is faced with stone, and at numerous places by the river-side stone platforms enclosed by piles are constructed. One such platform is 1100 yards long and 20 feet wide. At both ends of these platforms buttresses are constructed parallel to the river wall. The bore as it travels along the banks is deflected by these buttresses into the middle of the river, and the junk master is thus enabled to take shelter from the destructive force of the tidal wave. Within the shield of a platform his junk slides harmlessly up and down the face of the slope of the embankment.
   Working Models.—Within the last thirty years the practice of experimenting with working models of a tidal estuary before laying out new works for regulating it, has been initiated. Professor Osborne Reynolds’s paper before the British Association in 1887 first called attention to the utility of this procedure. The British Association in 1889 appointed a committee to investigate, by means of working models, the action of waves and
    1 "The Bore of the Tsien-tang-Kiang” (Commander W. U. Moore, R.N.), Proc. Insi. C. Vol. XCIX, pp. 297-304.