The influence of the tides on navigation requires no emphasis to those who have had the responsibility of navigating ships in tidal waters, but there will be few who will have entirely escaped some of the difficulties which tidal problems may bring, especially in unfamiliar waters. Most navigators will therefore have some sympathy for the experience of Alexander and his armies on the way to the mouth of the Indus. Their experience in the Mediterranean had not prepared them for what they had to suffer when they passed into tidal waters.