398 EEPOET— 1891. 



all, represents 1,743 million cubic yards on a 80-foot tide, or 437 million 

 on a 15-foot tide. This was distributed over 40,000 tides, or sixty years, 

 so that even with, the 15-foot tide it would represent 7 million cubic 

 yards a year. 



After the dredging the rise of tide fell from -081 to "073 foot, which 

 would result from the lowering the sand which was above low water. 



18. Experiments with Training Walls. Experiment V. (continued) with 

 Training Walls, Tanlcs E and F, from January 7 to Fehruarij 20, 1891. 

 Fla7i 4, Plate XII. — Having arrived at similar^^nal conditions of equili- 

 brium in tanks E and F, in which the sand was entirely below low 

 water from Section — 19 up the rivers (32 miles from the top of the 

 river) to the generators, and in which there were bars in the estuary 

 below the mouths of the rivers, reducing the depth of water at low tide 

 from 28 feet in the river to a minimum of 12 on the top of the bars, it 

 seemed an opportunity not to be lost for testing the similarity of the 

 effect in the two tanks of prolonging the rivers by training walls through 

 the bars. 



With this view walls of thick paper saturated with paraffin pushed 

 vertically into the sand and extending up to low water were run out 

 from the end of the river, preserving the same divergence as the walls of 

 the river to Section 22, or 40 miles on a 30-foot tide, the tanks being 

 stopped for the purpose. 



These walls produced no apparent effect whatever on the depth of 

 sand between the walls, during 20,000 or 30,000 tides. They were then 

 replaced at the upper end by walls of sheet zinc extending to half-tide, 

 which did produce an apparent effect, inasmuch as the sand accumulated 

 outside the walls, forming an apparent channel within; also the sand rose 

 in the river, doing away with the appearance of a bar. These effects 

 were similar in both models after 40,000 tides had been run. 



The old walls were removed in both tanks and replaced by walls 

 commencing at | tide at the mouths of the rivers, and falling during 

 the first 4 or 5 miles to half-tide, at which they were continued to 

 Section 22. 



In tank E the walls were advanced gradually from the mouth of the 

 river at a rate of about half a mile in 700 tides (year). The result of this 

 is shown in Plan 4, Plate XII., tank E. There is no improvement in 

 the navigable depth of the river. 



In tank F the walls were put in and then the tops of the ripple bars 

 were daily dredged off between the walls. This was continued for 

 100,000 tides, during which 5 per cent, of the tidal capacity was 

 removed, or about 1,000 million cubic yards on a 30-foot tide, or 250 

 millions on a 15-foot tide, which represents 7 millions annually on the 

 30-foot tide, or 1'8 millions on a 15-foot tide. The effect, as shown in 

 Plan 4, tank F, Plate XII., is to add some 20 feet to the depth on a 30- 

 foot tide, or 10 feet on a 15-foot tide. 



The silting up behind the walls is the same as in tank E, and the 

 detriment to the navigable depth of the river is also similar. 



19. Experiment V. {continued) with Tide deviating from the Simple Har- 

 monic in Tank E, Fehruary 23 to March 12. — This was meant as a pre- 

 liminary experiment. The balance of the generator was altered to give 

 a rise of tide in 17 seconds and a fall in 20. The experiment was run 

 for about 40,000 tides, and a survey taken, which showed little or no 

 effect. On carefully examining the tide curves it was found that they 



