This analysis, using existing basic hydro‐dynamic theory for the entrance conditions into slits, has resulted in easily provable equations in this approximation, valid for Newtonian liquids without the complication of inertia (Reynolds number). This assumption is proved to be valid for the flows discussed. Furthermore, the limits of the existing theory are delineated, especially the area of transition from the converging to the parallel slit flow. This inaccessible range can be elucidated by direct experiments or by numerical methods with computers. This paper is the introduction to a series of experimental investigations which further analyze the flow conditions of Newtonian or viscoelastic liquids.