332 HEPORT— 1889. 



parts of the tank. The want of means and time has prevented any 

 attempt to supply such a gauge. 



Curves have been obtained for most of the experiments by means of the 

 simple tide-gauge. The crank-wheel being divided into sixteen equal 

 arcs, one observer observes the wheel and another the gauge. When a 

 particular number comes to the index the observer at the wheel calls, and 

 the other observer reads the gauge, and then shifts the sliding pointer to 

 the point at which the tide-index was, so that on the next revolution, 

 when the call comes again, he can observe if the pointer coincides exactly 

 with the index or requires adjustment. Having brought about coinci- 

 dence, he then proceeds to the next number. In this way it takes about 

 half an hour to read the curve. Time, however, is not the only objection, 

 a greater one being that any irregularities in the motion of the wheel do 

 mot appear in the curve. The motion of the wheel has been as far as pos- 

 sible checked by the clock, but still there is room for important errors, 

 which a chronograph would obviate. 



The Selection of Sand. — Sir James Douglass having informed me that 

 clean shell sand could be obtained, and having sent me samples which, 

 from the tests to which I subjected them, seemed to be quite as readily 

 moved by the water as the finest Calais sand, I asked him to procure a 

 quantity — fifteen bushels of Huna Bay shell sand — and in the meantime 

 I procured a similar quantity of Calais sand, so that I might be prepared 

 with whichever showed itself best in actual experiments. 



Selection of the Experiments. — It having been decided that in the first 

 instance the purpose of the experiments should be the comparison of the 

 distributions of sand produced under particular lateral configurations, 

 and with different relations between the vertical and horizontal scales in 

 the same model, and with similar relations in these scales in the two 

 models, the only matters left for selection in starting these experiments 

 were the scales and particular configurations. 



There was apparently no reason for attempting the very difficulfe 

 operation of modelling any actual estuary, and, setting this aside, the 

 •question of choice mainly turned on whether it was best to begin with 

 complex or simple circumstances. There was considerable temptation 

 to commence with complex, i.e., boldly irregular boundaries, so that the 

 influence of the boundaries might predominate over such other influences 

 as exist ; in which case the influence of the boundaries would be tested by 

 the similarity of the distributions produced with diSerent ratios of hori- 

 zontal and vertical scales. On the other hand, however, it appeared that 

 as the main object of these researches is to differentiate and examine the 

 various circumstances which influence the distribution of the sand, it 

 was desirable, in starting, to simplify as much as possible all the circum- 

 stances directly under control, and so afford an opportunity for other 

 more occult causes to reveal themselves through their effects, and to 

 determine the laws of similarity of these effects. 



The simplest of all circumstances would be that of no lateral boun- 

 daries whatever — a straight foreshore of unlimited length with a shelving 

 sandy beach, up r.nd down which the tide runs until it has brought the 

 beach to a state of equilibrium. 



This being an impossibility, the nearest approach to it is that of a 

 beach or estuary with vertical lateral boundaries parallel to the direction 

 of flow of the tide. And the broader such estuary is in proportion to its 

 length the less would be the effect of the lateral boundaries. The effect 



