388 REPORT— 1891. 



after 32,000 ; while in the longer experiments farther surveys have been 

 made. With the spring and neap tide, the rate of action being much 

 the slower, intervals between the surveys have been longer. In all 26 

 complete surveys have been made, and 20 plans showing contours cor- 

 responding to every 6 feet reduced to a 30-foot tide, together with sections 

 and tide curves Plate III., are given in this report. 



The general conditions of each experiment, together with the general 

 results obtained, are shown in the table, while a description of each ex- 

 periment is given in § VI. 



The Committee have been fortunate in retaining the services of Mr. 

 Greenshields, who has carried out the experiments, observing and record- 

 ing the results, besides executing such modifications as have been required, 

 designing the compound harmonic gearing for the spring and neap tides, 

 which has answered excellently. 



Mr. Bamford has kindly continued his assistance in conducting the 

 investigations and reducing the results. 



§ II. — General Results and Conclusions. 



4. The conditions of eqiiilihrium with a long tidal river entering at the 

 top of a V-shaped estuary. — The experiments in tanks C and B made last 

 year led to the conclusion stated in Art. II of the Second Report : ' that 

 the effect of a river 50 miles long, when reduced to a 30-foot tide, increas- 

 ing gradually in width until it enters the top of a V-shaped estuary, is 

 entirely to change the character of that estuary. The time occupied by 

 the water in getting up the river and in returning causes this water to 

 run down the estuary while the tide is low, and necessitates a certain 

 depth at low water, which causes the channel to be much deeper at the 

 head of the cstuar}'. In its effects on the lower estuary the experiments 

 with the tidal river are decisive, but as regards the action of silting up- 

 the river further investigation is required, both to establi.sh similarity in 

 the models, and to ascertain the ultimate condition of final equilibrium.' 



From this year's experiments. III., IV., V., VI., and VII., in tank E> 

 and V. and VI. in tank F, it appears that if the length of the tidal river, 

 reduced to a 20 font tide, is 50 miles; or talcing ^ for the length of the 

 tidal river in miles and h for the rise of tide at the mouth of the estuary in 

 feet, if 



R=8-5V7t 



the river will Jceep open so that the tide will rise to the top, the sand falling' 

 gradually from the top of the river to the level of about mean tide at the 

 mouth. 



That the depth of wafer in the river and at the top of the estuary increases 

 rapidly with the length of the river, and luhen 



R=I2v/7i 



the level of the sand at the mouth of the river will he more than h feet helow 

 the level of low water and the hottom will he beloiv low water level for more 

 than half the length of the river above its mouth. 



5. The similarity of the results in the tanhs E and F. — The experiments 

 in the tanks E and F this year confirm those of last year in showing that 

 during the early stages of forming the estuary from sand at the level of 

 mean tide the action in the river is different in the small tank F from 



