When we mathematically model natural phenomena, there is an assumption concerning how the mathematics relates to the actual phenomenon in question. This assumption is that mathematics represents the world by “mapping on” to it. I argue that this assumption of mapping, or correspondence between mathematics and natural phenomena, breaks down when we ignore the fine grain of our physical concepts. I show that this is a source of trouble for the mapping account of applied mathematics, using the case of Prandtl’s (...) Boundary Layer solution to the Navier-Stokes equations. (shrink)

The evolution of the equation of mass conservation in fluid mechanics is studied. Following early hydraulic approximations, and progress by Daniel and Johann Bernoulli, its first expression as a partial differential equation was achieved by d’Alembert, and soon given definitive form by Euler. Later reworkings by Lagrange, Laplace, Poisson and others advanced the subject, but all based their derivations on the conserved mass of a moving fluid particle. Later, Duhamel and Thomson gave a simpler derivation, by considering mass flow into (...) and out of a fixed portion of space. The later propagation of these derivations in nineteenth-century British textbooks and treatises is also examined, including Maxwell’s on the kinetic theory of gases. (shrink)