Amidst the hurry of war and the rage for conquest Alexander always kept in view the future pacification of an invaded country; its products, therefore, were habitually ascertained and carefully noted, with a view to the increase of revenue and the development of commerce. But, beyond this, the great Macedonian conqueror, wherever he went, employed a numerous corps of scouts, and searchers, and men of science, to collect specimens of the curious animals, plants, and minerals to be found on the march. These he sent home from time to time to his great preceptor Aristotle, who was thus assisted to produce a work on Natural History which, for general accuracy of description and extent of knowledge, is a wonderful monument of scientific observation. When by the refusal of his soldiers to proceed further than the banks of the Hyphasis (the modern Beyah), Alexander found himself obliged to yield to their wish to be led back to Persia, he determined to sail down the Indus to the ocean, and from its mouth to proceed by the Erythrean Sea to the Persian Gulf, that a communication by sea might be opened with India. His intention was that the valuable commodities of that country should thus be conveyed through the Persian Gulf to the interior parts of his Asiatic dominions, and that by the Arabian Gulf they should be carried to Alexandria (the site of which he had most judiciously selected), and thence distributed to the rest of the world. With this object in view, he ordered a numerous fleet of boats and river-craft to be built and collected on the banks of the Hydaspes, at Bucephalia (either the modern Jhelum, or Jubalpore, some eighteen miles lower down the stream), and, when nearly two thousand vessels of various shape and size had been got together, he commenced his voyage down the Hydaspes to the Indus. The conduct of the flotilla was committed to Nearchus, an officer worthy of that important trust, though Alexander himself accompanied him in his navigation down the river. The army numbered a hundred and twenty thousand men and two hundred elephants. One third of the troops were embarked on the boats, whilst the remainder, marching in two columns, one on the right, and the other on the left side of the river, accompanied them in their progress. Retarded by various military operations on land, as well as by the slow advance of such a fleet as he conducted, Alexander did not reach the sea until more than nine months after the commencement of his journey. Having safely accomplished this arduous undertaking, he led the main body of his army back to Persia by land. The command of the fleet, with a considerable body of troops on board of it, remained with Nearchus, who, after a coasting voyage of seven months, brought it safely up the Persian Gulf into the Euphrates.