Recent uses to which tide records have been put has revealed that modern analytical techniques have outstripped the quality of available data. Lack of knowledge of the response of the conventional type tide well has been a serious handicap to workers in this field. The analysis in this paper shows that non-linearities, caused by the flow through the orifice, produce the main features of the response of the system; the hydrodynamic filtering of waves, due to the depth of the orifice and the effect of the inertia of the water in the well, are relatively unimportant.