As was the case under Justinian's predecessors, the Empire's economic health rested primarily on agriculture. In addition, long-distance trade flourished, reaching as far north as Cornwall where tin was exchanged for Roman wheat. Within the Empire, convoys sailing from Alexandria provided Constantinople with wheat and grains. Justinian made the traffic more efficient by building a large granary on the island of Tenedos for storage and further transport to Constantinople. Justinian also tried to find new routes for the eastern trade, which was suffering badly from the wars with the Persians.