The inhabitants did not neglect the advantages bestowed upon them by nature. A system of navigable canals that may excite the admiration of even the modern engineer, connected together the Euphrates and Tigris, those great arteries of her commerce. With a skill, showing no common knowledge of the art of surveying, and of the principles of hydraulics, the Babylonians took advantage of the different levels in the plains, and of the periodical rises in the two rivers, to complete the water communication between all parts of the province, and to fertilise by artificial irrigation an otherwise barren and unproductive soil. Alexander, after he had transferred the seat of his empire to the East, so fully understood the importance of these great works, that he ordered them to be cleansed and repaired, and superintended the work in person, steering his boat with his own hand through the channels. High-roads and causeways across the Desert united Syria and Palestine with Babylonia. Fortified stations protected the merchant from the wandering tribes of Arabia, walled cities served as resting-places and store-houses, and wells at regular intervals gave an abundant supply of water during the hottest season of the year. One of those highways was carried through the centre of Mesopotamia, and crossing the Euphrates near the town of Anthemusia led into central Syria. A second appears to have left Babylon by the western quarter of the city, and entered Idumæa, after passing through the country of the Nabathæans. Others branched off to Tadmor, and to cities which were built in the midst of the Desert almost solely for purposes of trade. To the east of Babylonia was the celebrated military and commercial road described by Herodotus. It led from Sardis to Susa in ninety days journey, and was furnished, at intervals of about fifteen miles, with stations and public hostelries, probably resembling the modern caravanserais of Persia. Merchandise and travellers descended the rivers upon rafts of skins, as well as in boats built of reeds coated with bitumen, or of more solid materials. The land trade was no doubt principally carried on, as at the present day, by caravans of merchants, who loaded their goods on the backs of camels, horses, and asses.