L.F. Richardson, The Approximate Arithmetical Solution by Finite Differences of Physical Problems Involving Differential Equations, with an Application to the Stresses in a Masonry Dam, Proceedings of the Royal Society of London, 1910, available here.
From the Abstract,
The object of this paper is to develop methods whereby the differential equations of physics may be applied more freely than hitherto in the approximate form of difference equations to problems concerning irregular bodies. (... ...).
Both for engineering and for many of the less exact sciences, such as biology, there is a demand for rapid methods, easy to be understood and applicable to unusual equations and irregular bodies. If they can be accurate, so much the better ; but 1 per cent, would suffice for many purposes. It is hoped that the methods put forward in this paper will help to supply this demand.
According CFD-Online, this 50 page paper is a key paper in Computational Fluid Dynamics.