RIVERS OF NEW SOUTH WALES. 401 



the first pl.ace, the rainfall on the higli lands is greater than on 

 the plains ; in the second place, the d(>bris washed down from the 

 hills, and deposited near them is composed of boulders, pebbles, 

 and heavy particles generally, which readily pei-mit undei-ground 

 percolation ; and, in the third place, owing to the denudation of 

 the rocks of which the hills are mainly composed, fi'ee access for 

 water is frequently given at the edge of the outcrop. 



Although the conditions of occurrence of underground sup- 

 plies of water have long been known, an extraordinary amount 

 of popular misundei'standing still exists on this subject. As an 

 instance T may mention that I have been gravely informed by 

 intelligent as well as successful men of business, that certain 

 persons merely by inspecting a plot of land in our western plains, 

 can tell whether water is to be found there by deep boring. Mr. 

 C. S. Wilkinson has done much to dispel such mistaken ideas and 

 thrown light on the subject ; but it is not yet sufficiently well 

 known that in regard to the probability of finding underground 

 supplies of water, the only true diviner is the geologist, and that 

 the divining I'od of common sense is geological information and 

 experience. 



In considering the object of underground supplies of water, it may 

 be remarked at the outset that, as a general rule, all such large 

 supplies of fresh water are streams and not merely reservoirs. On 

 the other hand, if the water be salt the probability is that it is 

 stagnant or nearly so. These are points which are frequently 

 overlooked, although on reflection their correctness is evident. 

 Another matter which is generally lost sight of is the fact that 

 the flow of water underground takes place under conditions which 

 are subject to common hydraulic laws. To illustrate this, take 

 the case of a twelve-inch pipe, five miles in length, and with a fall 

 of two hundred feet. We know that if for such a pipe we sub- 

 stitute as many pipes of two inches in diameter as would give the 

 same internal sectional area the water would flow with considerably 

 less velocity than in the twelve inch pipe, and the discharge would 

 be less in a corresponding degree. If, still preserving the same 

 sectional area of waterway, we substitute one inch pipes, the 

 velocity and discharge are still further diminished. To take an 

 extreme case, suppose that instead of the original twelve inch pipe, 

 we take one of such diameter that when filled with sand, the 

 interstices in the sand would be equal to the sectional area of the 

 twelve inch pipe. We know that even with a fall of two iiundied 

 feet in five miles the velocity in this case would be small ; but 

 still if a vertical piece were added at the lower end of the pipe 

 the water would rise in it to a height of two hundred feet, minus 

 the height due to the resistance. Now if the height and length be 

 each multiplied by ten and the sand be considered as covered with 

 clay and soil, instead of being inclosed in a pipe, the case bears no 



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