Empedogles' simile of the clepsydra (DK6 31B100) is a crucial document for historians of ancient science. It has been much discussed, and often quoted in evidence, in spite of formidable differences of opinion about its significance. ‘Empedocles undertook an experimental investigation of the air we breathe’ (B. Farrington). ‘The star example of a physical “experiment” in the natural philosophers, the clepsydra, was not an experiment at all, in the proper sense of the word’ (G. Vlastos). ‘All Empedocles did was to draw the explicit inference: “the vessel cannot be simply empty: the air in it cannot be nothing at all”. He did not invent the clepsydra in a laboratory’ (F. M. Cornford). The simile ‘ha tutto il carattere di una esperienza scientifica’ (A. Traglia). Now whether the fragment describes an experiment or not, it is certainly a simile, and the first step must be to understand the force of the simile. It is possible, in my view, that the differences of opinion about the fragment spring from various misunderstandings of the simile; and I propose in this article to offer an explanation of its details which I think is new and which may enable us to form a clearer picture of its place in the history of science.