6S Remarhs on the 'Beport of tlie Water Commission. • 



pecuniary difficulties wtich put an end to the undertaking." 

 Though this work was not comj^leted, the eminent authority that 

 refers to it not only does not ridicule it as impossible but speaks 

 of it as quite practicable. 



Assuming, then, that the dam once made as proposed 

 would be stable, the second question has to be considered : 

 Would it prevent the intermixture of the salt and fresh 

 water, and would the river once sweetened be kept sweet ? 

 I think it would. It will be remembered that the normal con- 

 dition of affairs will be this — that the sweet water Avould stand 

 at the higher level. As long as water was running over the weir 

 it would stand one foot higher than high water springs, and seven 

 feet higher than low water neaps. If there were any percolation 

 at all, therefore, the feesh water would run into the salt, and not 

 the salt into the fresh. The stronger pressure would overpower 

 the weaker. All along the sea coast we have frequent illustra- 

 tions of the fresh water being close to the salt without their 

 intermixing. Wells sunk in the beach above high-water mark 

 almost always are found to contain fresh water. Where creeks 

 run out into the sea it is to be noticed that a sand-bar is thrown 

 up at the mouth when the creeks are not ilowing. The inland 

 water is quite fresh, although the salt is only a few yards off, and 

 there is nothing but a sandbank between them. The water 

 pumped into the city from Botany is within a few yards of the 

 salt water, and separated from it only by a rocky ridge and a 

 sandbar. In the case of the proposed dam there would not only 

 be sand, but stone between the two waters, and this stone would, 

 in course of time, tend to become very firmly consolidated. Sir 

 John Eennie, describing the Plymouth breakwater, says, " That 

 80 solid had the work become, that it appears to be but one huge 

 stone," and " whenever excavations are required they can only 

 be made by quarrying in the usual way." These excavations 

 could only have been wanted near the surface, and we may infer 

 from this how solid the work had become lower down. A rubble 

 breakwater is of course in the first instance porous, but as it 

 settles down and the interstices get filled up it tends to become 

 more and more like a solid mass. 



In works of this description, we need not be above taking 

 hints from the engineering of Eastern nations, who had learned 

 a great deal by practical experience before Western science was 

 born. In a paper on Irrigation in India, read by Mr. Allan 

 Wilson before the Institution of Civil Engineers, the writer gives 

 some interesting statements as to the construction of river dams 

 in India. A few of these remarks I will quote, as they are 

 singularly apropos to our present subject. 



