It was a simple mathematical formula, devised by a nineteenth-century Frenchman, to compute the flow of water through porous media. The law stated that the quantity of water (Q) flowing through a given area is equal to the hydraulic conductivity (K) of the material through which it flows, multiplied by the size of the opening (A), multiplied again by the gradient, or angle of incline (I). Thus: Q = K × A × I. Once one knew what values to put into the formula, it was quite easy to work out. Unlike Guswa’s splendid three-dimensional computer model, it required only a pencil and paper.

Jonathan Harr, A Civil Action, loc. 2152