The first merchants who crossed the Indian ocean, embarking in small ships of light draught, timidly hugged the shore during their whole voyage, dipping into every bight for fear of losing sight of land. But in the reign of Claudius a navigator named Hippalus discovered the monsoons, and noted their stability as to force and direction at certain seasons of the year.[766] Thenceforward the merchants, furnishing themselves with larger vessels,[767] boldly spread their sails to the wind, ventured into mid-ocean, and made a swift and continuous passage from the southern coast of Arabia to some chosen port in the vicinity of Bombay.[768] Such was the southern, and, within the Christian era, most frequented trade route between the Roman Empire and the Indies. There were, however, two other avenues, more ancient, but less safe and less constant, by which merchandise from the far East, mainly by inland transit, could enter the Empire. By the first of these, which traversed many barbarous nations, the eastern shores of the Euxine were brought into communication with northern India through the Oxus, the Caspian Sea, and the Cyrus. From a bend in the latter river, the emporium of the trade, the town of Phasis, was easily attainable.[769] The second, intermediately situated, was the most direct and facile of the three, but, as it lay through the Persian dominions, the activity of commerce by this route depended on the maintenance of peace between the two empires.[770] The Byzantine government, jealous of the intercourse of its subjects with their hereditary enemies, fixed Artaxata, Nisibis, and Callinicus[771] as marts beyond which it was illegal for Roman merchants to advance for the purposes of trade on this frontier.[772]
In the sixth century the Ethiopian kingdom of Axume,[773] nearly corresponding with Abyssinia, became the southern centre of international trade; and its great port of Adule was frequented by ships and traders from all parts of the East.[774] Ethiopian, Persian, and Indian merchants scoured the Gangetic Gulf, and, having loaded their vessels with aloes, cloves, and sandalwood, obtained at Tranquebar and other ports, returned to Siedeliba or Ceylon[775] to dispose of their goods. There transhipments were effected, and sapphires, pearls, and tortoise-shell, the chief exports of that island, were added to the cargoes of ships westward bound. In the same market a limited supply of silk was obtained from such Chinese merchants as were venturesome enough to sail so far.[776] From Ceylon such vessels voyaged along the Malabar coast between Cape Comorin and Sindu, near the mouth of the Indus, receiving on board at various places supplies of cotton and linen fabrics for clothing, copper and rare woods, together with spices and aromatics, musk, castor, and especially pepper. In the harbours of that seaboard they also met with the merchants from Adule, most of whom sailed no farther, and provided them with the freight for their homeward voyage.[777]