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HARKER : PRESIDENTIAL ADDRESS 
My meaning will perhaps be clearer if I expand this latter question 
a little. The geometrician, once satisfied of the validity of a -few 
simple axioms, can build up one theorem upon another by purely 
deductive reasoning, without any further appeal to experience. This 
is the extreme case ; but among the sciences dealing with the material 
world there are some in which mathematical precision and deductive 
reasoning find an important place. The mineralogical chemist, for 
example, has the law of molecular proportions, and (within limits) 
will reduce his analysis of a mineral to conform with this law ; in 
other words, he can use theory to correct observation. The law of 
rational indices affords like guidance to the crystallographer, and 
many other instances might be cited. There is, I think, no department 
of geology proper in which such a degree of certainty and exactitude 
has yet been attained. What the future may have in store I will not 
venture to predict ; but, so far as the immediate prospect is concerned, 
it seems that the only safe course for the geologist is that which makes 
continual appeal to observation, wherever such control is possible. 
By many this will be regarded as a mere common-place, but it 
would be easy to give reasons for insisting on it. Five years ago a 
distinguished physicist undertook to combat w^hat he termed " a 
curious obsession as to a matter of fact '"' on the part of geologists. 
While the text-books continue to repeat that the flow of a river is 
most rapid on the outer side of a bend, the fact is, he said, that it is 
most rapid on the inner side. This pronouncement, which can be 
confuted by any boy who has ever gone boating, was not made on the 
strength of any observations, but as the result of calculation applied 
to some ideal case. That was how the river ought to flow. It is true, 
there was also a reference to the behaviour of a wooden modeL a 
subject to which I will return later. It is not my purpose to discuss 
the phenomena of the flow of rivers, and indeed the matter is less 
simple than appears at a glance ; but I have cited the incident for its 
bearing on the relations between geology and the exact sciences. On 
one point doubtless all geologists will agree ; that, when there is a 
conflict between deduction and observation, we must prefer the 
evidence of our eyes to the most cogent demonstration. Some will 
perhaps go farther, and, while Avelcoming help from any quarter, will 
hint that, in relation to such a study as physical geology, mathematics 
is a good servant but a bad master. 
The truth is that such a problem as this is, from the point of view 
of analytical treatment, one of extreme complexity. It often involves 
