Later the “astrolabe,” an instrument almost equally crude, was introduced. It was made of a heavy tin or bronze plate, circular in shape, and pivoted to its centre was a bar running across it from side to side. It was marked in degrees and fractions, and while one man held it, as steadily as he could, a second sighted along the pivoted crossbar and a third read the angles. Vasco da Gama used one of these in 1497 on his voyage around the Cape of Good Hope, but it did not turn out to be much of an improvement on the cross staff. But up to this time, and even later, the science of navigation consisted almost solely of the approximate determination of latitude and mere guesses, based on the estimated speed and direction of the ship through the water, for longitude. So hopeless did it seem at that time for mariners scientifically to determine their longitude that an old writer on the subject is quoted by the Encyclopædia Britannica as saying, “Now there be some that are very inquisitive to have a way to get the longitude, but that is too tedious for seamen, since it requireth the deep knowledge of astronomy, wherefore I would not have any man think that the longitude is to be found at sea by any instrument; so let no seamen trouble themselves with any such rule, but (according to their accustomed manner) let them keep a perfect account and reckoning of the way of their ship.” These early sailors learned, of course, that their latitude could be worked out by observing the North Star, and they used that method, crudely, of course, but similarly to the way it is used to-day. For this a contrivance called a “nocturnal” was adopted. With this they could determine what position the North Star was in, in reference to the true pole, for, of course, the North Star does not exactly mark the pole, but revolves about it in a small circle. While the voyage of Columbus did not actually begin the era of discovery, it did greatly increase interest in exploration, and as most of this exploration necessitated long ocean voyages the interest in navigation grew apace. One of the earliest writers on navigation was a man named John Werner. In 1514 he explained the use of the cross staff, which for many years had been used on shore but had been first utilized at sea not very many years before Werner wrote. A little later one R. Gemma Frisius wrote a book which contained a great deal of information useful to men of the sea. In it he described the sphere with its parallels of latitude and its meridians of longitude much as we use them to-day. Up to this time, however, no agreement had been made upon what meridian to base the measurement of longitude. Nowadays the meridian of Greenwich is used. Frisius, however, suggested the meridian of the Azores. Any meridian, of course, would do, provided that the necessary data be based upon it, but the data available in the early 16th Century were slight indeed.