Reconstruction of a prismatic prototype of cylindrical inflow clocks. Operating with the simplest fractions, 1/2 and 1/3, and drawing straight lines only, we may construct a prismatic diagram which accounts for the unusual features of the cylindrical diagram of the Edfu inflow clock. The new diagram points to an Egyptian--not Greek-- origin of several passages of classical literature dealing with the rate of increase of the length of the day. Reproduction and discussion of hitherto unpublished items connected with water clocks, such as the prismatic model in the Metropolitan Museum (Acc. No. 86.1.93) and the Medinet Habu astronomical ceiling. It is shown that the decanologue of the Karnak outflow clock is closer related to the Senmut decanologue than to those of the Ramesseum and of Medinet Habu.