G. H. KNIBBS. XLIII. 
from its general direction, a fact which however is not 
easily dealt with, since we do not see what is taking place. 
From what has preceded, it will be seen, how largely the 
solution must be conjectural. In the scientific solution, 
however, the conjectures are not mere guesses; they are 
rather of the nature of hypotheses, which may be invoked 
to explain the facts. If these hypotheses can be so made, 
that, notwithstanding their uncertainty, to give a definite 
answer to questions concerning how far exploitation can 
be carried, they constitute a satisfactory and real solution 
of the problem. 
9. Thermal and other conditions.—The viscosity of 
‘water is increased by contained salts and diminished by 
heat. The absence of uniformity, however, between heat 
and depth is shewn by the very varied temperatures of 
artesian waters, often strikingly high, and makes an a 
priori numerical solution of the flow somewhat difficult. 
For steady motion, the viscosity of water greatly affects 
the flow, since this varies directly asthe fluidity or reciprocal 
of the viscosity, the relative values at different tempera- 
tures being as follows :— 
Deg. C. 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100” 
Fluidity 1°00 1°36 1°77 2°22 2°72 3°23 3°77 4°34 4°92 5°52 6°13 
In turbulent flow the viscosity has relatively very small 
influence, so that in the stratum the temperature effect is 
large, but in the pipe it is very small. 
10. Fluctuations of pressure: theoretical considerations. 
—G. H. Darwin’ estimated that a rise of barometric pres- 
sure of 1 inch over the whole of Australia, was sufficient 
to lower the entire continent from 2 to 3 inches, and that 
tides may produce by loading the surface a depression of 
 G. H. Knibbs—Journ. Royal Soc., xxv1., pp. 318-319, 1897. 
* Nature, Vol. xxvitl., p. 367. 
