The invention provides a variety of fluid mechanics devices such as fluid-flow valves; piston and cylinder devices; fluid-flow restrictors; fluid reservoirs and fluid-flow manifolds. The devices are build up from stacks of wafer-like components. Which preferably are transparent and in each case, at least one of those components comprises an axial port and a radial port communicating with the axial port, there being concentric annular recesses in the faces of the component to receive an outer "O" ring type static seal between two mating components, and an inner "O" ring dynamic seal which acts between the wafer component and a vale rod piston rod, spindle or like device extending through the aligned axial ports of all the wafer components. Beside provide the double seal (static and dynamic) the arrangement also permits of great versatility of assembly.