390 EEPORT 1891. 



or 400 million cubic yards, removed at the rate of 7 million cubic yards 

 a year. 



In most navigable rivers two processes have been going on — dredging^ 

 and land reclamation — the first tending greatly to improve the rivers and 

 estnaries, the second to deteriorate them so that any improvement has 

 been a question of balance. Where the rivers have improved they vrill 

 probably continue to improve so long as dredging goes on, but if the 

 dredging should stop, for example in the Thames, there would in all 

 probability be a gradual deterioration, possibly ending in the silting up of 

 the tidal river. 



6. The effect of Tides deviating from the simple Harmonic Law. — One 

 attempt was made to study this question, when it was found tbat it would 

 require such modifications in the gearing as were not practicable in the 

 time, and so it was abandoned. 



7. The action of Tides varying from Spring to Neap. — The rates of 

 action and conditions of final equilibrium in rectangular tanks, in V-shaped 

 estuaries with a long tidal river, and in each estuary rendered unsym- 

 metrical by large groins, have been investigated with tides varying 

 harmonically from spring to neap, and again to spring in 29 tides. The 

 ratio of these at spring and neap being 3 to 2 as compared with uniform 

 tides, having the same rise as the spring tides, also for uniform tides 

 having the same rise as the mean of spring and neap, the results showing 

 definitely : 



(1) That the condition of Final Equilibrium in all cases ivith sprinrj and 

 neap tides xoas the same as that with miiforin tides having the same rise as 

 springs, and much greater, cssentialh/ different, from that ivith a uniform 

 tide having a rise equal to the mean rise of spring and neap tides. 



(2) That the Rate of Action loith the varying tide is much smaller than 

 that of a uniform tide having the rise of the spring tide. The ratios being 

 definite, abotd2'^ to 1. 



(3) Tliat the limits of similarity obtained for all spring tides hold 

 approximately for tides varying from spring to neap. 



8. The effects of prolonging the rivers into the estnaries by walls below- 

 high ivater. Experiments V. in tanks E and F having arrived at 

 similar final conditions of equilibrium (in which the depth of the rivers 

 for some distance above their mouths was reduced to a 30-foot tide, nearly 

 30 feet at low water, while the sand in the estuaries gradually rose from 

 the mouths of the rivers until it reached to witbin 12 feet of low water at 

 a distance of 14 miles below the mouth and then fell again, all the sand 

 being below this level, there being passes which formed a crooked deep 

 water channel), opportunity was taken to prolong the banks of the river 

 by walls at first up to low water and extending through the bar to a 

 distance of 44 miles from the mouths of the rivers. Then raising these 

 walls to half-tide, and finally carrying the walls forward slowly in tank 

 E at a rate of half a mile a year (700 tides), and in tank F dredging from 

 between the walls at a rate of seven million cubic yards a year (700 tides). 



This was done in the first place as a further test of the similarity of 

 the action in the two tanks, and secondly as affording an interesting- 

 study as to the effect of vertical walls in the direction of the current in 

 the bed of a tide-way. The effect of these walls at the level of low water 

 and at half tide were precisely similar in both tanks ; in neither case did 

 they produce any sensible effect at all on the level of the sand between 

 them. At the level of half- tide they caused in both tanks a slight silting- 



