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ARTESIAN WATER IN N. S. WALES AND QUEENSLAND. 433 
by the overflowing of numerous thermal springs, which is the 
principal source of the water supply of our Cretaceous artesian 
basins. During the long period between the sealing of these basins 
and the formation of the superincumbent strata, the action of the 
earth’s contraction must have induced a certain degree of lateral 
pressure. In consideration that the weight of ordinary sandstone 
is 150 ibs. per cube foot, it would require but little thickness of 
overlying strata to induce, in conjunction with the vast indefinable 
dynamical action of lateral pressure, all the propelling force 
evidenced by artesian flow independent of any other agency. 
Mr. G. H. Knisps—In accounting for the velocity of flow from 
artesian wells, said the theory of expansion of the water by heat can 
only apply where the water is actually imprisoned, as assumed by 
Captain Gipps. Its effect moreover must soon pass off, and con- 
tinued flow would have to be accounted for in some other way, as 
by superincumbent pressure. If, however, there be a point of 
intake where the water-bearing stratum is covered only by perme- 
able strata, the heat-expansion cannot raise the effective head 
above that point. In considering the efficiency of the head it is 
necessary to distinguish between ‘hydraulic’ and ‘hydrostatic’ 
head and pressure. Were there no outlet, the hydrostatic head, 
2.€., the elevation of 1,000 or 1,100 feet of the point of intake, 
would more than account for any known flow in the artesian wells 
of the Colony. But if there be an outflow, the head will be 
hydraulic and not hydrostatic. Then assuming that the sectional 
area of the region of subterranean flow is constant, the hydraulic 
gradient will be a curve with its convexity upwards, provided 
that the condition of increasing resistance to flow by diminution 
in the porosity of the stratum as the point of outflow is approached, 
is as postulated by Professor David, a geological fact. For the 
hydraulic head is expended not only in producing the velocity of 
outflow, but also in overcoming resistances to flow. This result 
of hydro-dynamic theory, it seems to me, is correctly and interest- 
ingly illustrated by Prof. David’s apparatus. In regard to the 
effect of superincumbent pressure, it cannot promote flow except 
B s—Dec. 6, 1893, ] 
