Description:One important result of this work is a resurgence of interest in the relationship between the
differential equations that express mechanical laws and the boundary conditions that constrain the
solutions to those equations. However, many of these accounts miss a crucial set of distinctions
between the roles of mathematical boundary conditions modeling physical systems, and the roles
of physical conditions at the boundary of the modeled system. In light of this systematic oversight,
in this dissertation I show that there is a difference between boundary conditions and conditions at
the boundary. I use that distinction to investigate the roles of boundary conditions in the models
of fluid mechanics. I argue that boundary conditions are in some cases more lawlike than
previously supposed, and that they can play unique roles in scientific explanations.