Fluid flow within the streamline (viscous) range, made visible by appropriate means, may be made to simulate fields occurring in electrostatic, magnetic, electric current, heat flow, chemical diffusion, and other situations. Simple, reliable, and inexpensive techniques for setting up the desired fluid flow situations have been worked out, and are described herein. Two-dimensional fields with one, several, or many non-distributed sources (or sinks) may usually be made up and operated with ease. Techniques for the simulation of certain three-dimensional fields with axes of symmetry have also been developed. Through the invention of the ``sandbed'' feature, the fluid flow method has been greatly extended: fields due to distributed sources can be simulated, not only outside the sources, but inside them as well. Numerous photographs of fluid mappers in operation, illustrating all of the foregoing types of cases, are included.