LIGHTSHIPS OWE THEIR ORIGIN TO THE ANCIENT ROMANS     Lightships, as we commonly know or remember them, have been around little more than two centuries, though a prototype existed in the ancient world. During the last few centuries B.C., Roman galleys carried at their mastheads open framework baskets in which a fire sometimes was built, serving as a signal light. Manned by an armed crew, such vessels patrolled the Roman coasts to guide and protect incoming vessels by providing a beacon and to deter piracy by showing that a warship was at hand. But, since the prudent Roman sailor tried to avoid nighttime voyages whenever possible, the first lightships never attained the importance of their successors.     By the 18th century, however, maritime commerce had become a 24-hour-a-day undertaking, with ships ranging the entire globe. In 1731, Robert Hamblin, an Englishman, obtained permission from King George II to outfit what would become the first modern lightship. His single-masted vessel was given the name Nore and took up its position a year later in England at Nore Sands in the Thames estuary. Resembling a small fishing sloop, the Nore carried two ship's lanterns, hung 12 feet apart from a cross arm high above the deck wherein burned flat wicks in oil. The Nore's log lists several accounts of almost futile struggles to keep the lanterns lit during any appreciable strength of wind, still, ship's masters considered the lightship a godsend, and similar vessels soon entered similar service off the coasts of most every seafaring nation.