PERMIT a brief correction to the paragraph in NATURE of April 7, p. 479, on the casts presented to the Science Museum. The variable divisions of the water-clocks are not for different lengths of day, but compensations for the changes of viscosity of water, over 9° and 12° F. respectively. This is proved by the extremes being nearer to the equinoxes than to the solstices, to harmonise with the slow passage of heat through massive temples. Further, the conical form of the clepsydra of 1400 B.C. was to compensate for the greater flow under fuller pressure, the form being a near approach to a portion of a parabola. Thus the variation of pressure was as 1 : 3.7, and the water varied as 1 : 2.9 to meet this.