the body; these two tubes, marked respectively D D and E, are connected by the sliding tube F, which is attached to the two racks F and G, into which are geared the two toothed wheels B and C. When the knife is passed from P to O it first rotates the holding-on wheel A, and then strikes against the radial face of the wheel C, turning it through a small arc, thereby moving the racks, and, sliding the connecting tube F out of D, allowing the knife to pass, which next strikes the radial face of the wheel B, and, by turning it, restores the sliding connecting tube F into D, and thus recompletes the connection. The sucking-up the liquid being accomplished in a similar manner to that in the drinking bird already described.[6] [Illustration: Fig. 13.] I have now done with Hero of Alexandria, but, before passing to another period, I cannot resist showing you an invention of his which although not an automaton is too interesting in the light of modern civilization to omit. This (Fig. 14) is Hero’s automatic penny-in-the-slot machine for giving a drink in exchange for a coin. If a “coin of five drachmas” be dropped into the slot it falls on a little plate at the end of a lever thereby opening a valve and allowing the liquid to escape through the nozzle. [Illustration: Fig. 14.] It is more than probable that Hero was not himself the inventor of all the devices he describes, it is possible that many are due to Ctesibius whose pupil he was, and it is clear, from his own writings, that he was acquainted with the writings of Philo and of Archimedes. He was, however, the first to _describe_ these inventions, and therefore it is only fair, in the absence of other evidence, to give him the credit. [Illustration: Fig. 15.]