It is clear that the prevailing winds must now have been studied with care, and the intercourse between West and East so arranged as to admit of the utmost possible advantage being derived from them. Towards the latter end of July the annual, or Etesian (north), wind commences its influence and extends from the Euxine Sea to Syene in Upper Egypt.[232] As a northerly wind, prevailing at the time of the year when the Nile is at its greatest height, it affords an excellent opportunity of advancing against the stream. Hence the voyage from Alexandria to Coptos, a distance of three hundred and eight Roman miles, was usually performed in twelve days.[233] The Canopic branch of the Nile (the nearest to Alexandria) was then the chief navigable approach to Egypt from the sea. From its entrance a canal had been formed connecting it with Alexandria, so that traders from Coptos could pass through it without landing their cargoes at Alexandria, and without paying the custom-dues exacted from other vessels. It is worthy of remark, that though Berenice was built by Philadelphus, the real value of its position was not at first recognised; nor was it fully appreciated till the discovery of the regular uniformity of the winds at certain seasons of the year gave a special impetus to the trade with the far East. It is needless to inquire when the monsoons were first noticed: it is enough to know, that even if the principle of them was ascertained at a remote period, little or no use was made of this knowledge by the Alexandrian merchants till about the reign of Claudius, and of the successful voyage of Hippalus.[234] The Romans perceiving the great advantage Hippalus had made known by his adventurous voyage justly named the monsoon wind after him.