trading. The Roman Empire traded with the Chinese (via Parthian and other intermediaries) over the Silk Road. Sea routes Maritime archeology and ancient manuscripts from classical antiquity show evidence of vast Roman commercial fleets. The most substantial remains from this commerce are the infrastructure remains of harbors, moles, warehouses and lighthouses at ports such as Civitavecchia, Ostia, Portus, Leptis Magna and Caesarea Maritima. At Rome itself, Monte Testaccio is a tribute to the scale of this commerce. As with most Roman technology, the Roman seagoing commercial ships had no significant advances over Greek ships of the previous centuries, though the lead sheeting of hulls for protection seems to have been more common. The Romans used round hulled sailing ships. Continuous Mediterranean "police" protection over several centuries was one of the main factors of success of Roman commerce, given that Roman roads were designed more for feet or hooves – with most land trade moving by pack mule – than for wheels, and could not support the economical transport of goods over long distances. The Roman ships used would have been easy prey for pirates had it not been for the fleets of liburna galleys and triremes of the Roman navy. Bulky, low-value commodities, like grain and construction materials, were traded only by sea routes, since the cost of sea transportation was sixty times lower than land. Staple goods and commodities like cereals for making bread and papyrus scrolls for book production were imported from Ptolemaic Egypt to Italy in a continuous fashion. The trade over the Indian Ocean blossomed in the 1st and 2nd century AD. The sailors made use of the monsoon to cross the ocean from the ports of Berenice, Leukos Limen and Myos Hormos on the Red Sea coast of Roman Egypt to the ports of Muziris and Nelkynda in the Malabar Coast. The main trading partners in southern India were the Tamil dynasties of the Pandyas, Cholas and Cheras. Many Roman artifacts have been found in India; for example, at the archaeological site of Arikamedu, in Puducherry. Meticulous descriptions of the ports and items of trade around the Indian Ocean can be found in the Greek work Periplus of the Erythraean Sea (see article on Indo-Roman trade). Standard weights and measures A standard amphora, the amphora capitolina, was kept in the temple of Jupiter on the Capitoline Hill in Rome, so that others could be compared