These graduations indicated the hours. He also made the boat turn a wheel, which at each hour caused a number of balls to fall into a silver goblet. The number of balls indicated the number of hours, so that the clock struck the hour very much like our clocks of to-day. Heron is famous for having invented a kind of steam-engine, consisting of a hollow ball with two nozzles pointed in opposite direction (Fig. 7). The steam passed into the ball through the arms by which it was suspended, and the steam hitting against the air outside caused the ball to spin round. In another form of the apparatus (Fig. 8) a ball was kept jumping up and down by the steam issuing from a vertical pipe. Heron found that when a narrow tube is put into water, and the upper end is stopped with the finger while still under water, then on lifting the tube out of the water with the stopped end uppermost, the water does not flow out at the open end below until the finger is removed. He, therefore, constructed a vessel (Fig. 9) which became known as the “Vestal’s Goblet”; it was filled by plunging it into water, stopping the upper opening with the finger, and lifting it out. The water escaped in a spray through the small holes below on removing the finger. [Illustration: Fig. 9.--The Vestal’s Goblet.] [Illustration: Fig. 10.--The Double Goblet.] In another form of the instrument there were two compartments which were filled in the same manner with wine and water respectively. It was a custom among the Greeks to mix wine with water, and offer the same mixture to the gods in the temples. Heron also made a self-feeding wick for oil lamps, which were then in common use. The oil was burnt in an open vessel (Fig. 11), and as it burnt away the wick was consumed also, but Heron made the oil itself turn up the wick. He made a plate of wood float on the oil; as the oil burnt away the plate of wood gradually sank, and in doing so it moved a cog wheel. The cog wheel in turning moved a straight rack, or toothed bar of wood, at the end of which the wick was fastened. In this way the wick was moved on as the oil was consumed, and the lamp was kept steadily burning. The most important instrument which Heron improved was the Surveyor’s Level. Egypt depends for its fertility upon the floods of the Nile, as there is very little rain in Egypt. [Illustration: Fig. 11.--Heron’s Self-Feeding Wick for Lamps.] From the oldest times the Egyptians had to see that they could distribute the land after the flood in the same way as it was distributed before, each man getting his own land back. This was found to be very difficult, as many landmarks were destroyed by the flood. The art of Surveying was practised at a very early date in Egypt, and there is no doubt that the science of Geometry was born in Egypt, as it was necessary in order to be able to determine the extent and boundaries of land. [Illustration: Fig. 12.--Modern form of Heron’s Level.]