An important problem in connection with Surveying was to find when two points are on the same level. This is done nowadays with a spirit-level if the points are close together, or with an instrument called a theodolite if the points are far apart. But a theodolite requires a telescope, and the Alexandrians lived long before the telescope was invented. Heron, however, got over the difficulty by constructing a long box filled with water and provided with openings, in the form of a cross, at each end. Above the crosses were glass tubes communicating with each other, in which the water was kept at the same level, and when that was the case the two crosses were exactly at the same level, and any object seen through both crosses together was at the same level as the crosses themselves. Heron probably used rods very similar to the black and white rods used by surveyors at the present day for measuring distances and levels. Heron also invented a mechanical stone crusher, an organ driven by water power, a counting machine to show the distance travelled by a ship or a chariot, and an arrangement of mirrors which he called a spy-glass. Many of the works of Heron have been preserved to us. They are written in Greek, which was the language used in Alexandria since its foundation. Heron was always careful to make his reasoning quite clear to his pupils, and he insisted that they must not believe anything without proof. “It is necessary,” he said, “that those who wish to become acquainted with mechanical art should know what causes are at work in every motion. It is important that nothing should be put before students without proof, and that nothing should remain doubtful for them. In our presentation every problem is to find a solution. We therefore recall various principles taught by the ancients which are connected with our subject.” CHAPTER III ARABIAN DAYS TO the east of the Red Sea there is a land called Arabia. Part of it is a desert over which a wind, called the simoom, blows its suffocating blasts. Another part is fertile mountain land, where the red anemone blooms. The third part adjoins the Indian Ocean and the Gulf of Persia. This part is rich and green, and planted with date-palms and coffee shrubs. From Arabia comes the Arab horse, small and grey, but beautiful to behold. It can go for forty-eight hours without a drink. The Arabs ride it without a saddle or stirrup, and only guide it by the pressure of the knee and a kind word. Its greased hoofs traverse the Arabian desert from end to end.