252 NAVIGATION a fine cord or wire placed at the supposed latitude. The second circle EQ is fixed at right angles to the first, and represents the equinoctial line. The upper side is divided into twenty-four parts, representing the hours from noon or midnight. On the inner side of that circle are marked the months and weeks. The third ring CC is attached to the first at the poles, and revolves freely within it. On the interior are marked the months, and on another side the corresponding signs of the zodiac ; another is graduated in degrees. It is fitted with a groove which carries two movable sights. On the fourth side are twenty-four unequal divisions (tangents) for measur ing heights and dis tances. Its use is illustrated by twenty problems ; it can do roughly all that any instrument for taking angles can. Thus, to find the latitude, set the sights C, C to the place of the sun in the zodiac, and shut the circle till it cor responds with 1 2 o clock. Peep through the sights and alter " the point of suspension till the greatest elevation is attained ; that time will be noon, and the point of suspension will be the latitude. The figure is slung at lat. 40, either north or south. To find the hour of the day, the latitude and declination being known : the sights C, C being set to the declination as before, and the suspen sion on the latitude, turn the ring CC freely till it points to the sun, when the index opposite the equinoctial circle will indicate the time, while the meridional circle will coincide with the meridian of the place. In this we may see the germ of the " equatorial " now used in the principal observatories. There is at present in the museum attached to the Royal Naval College at Greenwich an instrument, ticketed as Sir Francis Drake s astrolabe, prior to 1570. It is not an astrolabe, but may be a combination of astronomical rings as invented by Gemma with other things, probably of a later date. It has the appearance of a large gold watch, about 2| inches in diameter, and contains several parts which fall back on hinges. One part is a sun-dial, the gnomon being in connexion with a graduated quadrant, by which it could be set to the latitude of the place. There are a small compass and an hour circle. It is very neat, but too small for actual use ; it may be simply an ornament representing a larger instrument. There is a table of latitudes engraved inside one lid ; that given for London is 51 34 , about three miles too much. In 1537 Pedro Nunez (Nonius), cosmographer to the king of Portugal, published a work on astronomy, charts, and some points of navigation. He recognized the errors in plane charts, and tried to rectify them. Among many astronomical problems is one for finding the latitude of a place by knowing the sun s declination and the altitude when on two bearings, not less than 40 asunder. Gemma did a similar thing with two stars ; therefore the problem now known as a " double altitude " is a very old one. They could do it by the globe within a degree. To Nunez has been erroneously attributed the present mode of reading the exact angle on a sextant, the scale of a barometer, and various other things, the credit of which is due to Vernier nearly a hundred years later. The mode of dividing the scale which Nunez published in 1542 was the following. The arc of a large quadrant was furnished with forty-five concentric segments, or scales, the outer one graduated to 90, the others to 89, 88, 87, &c., divisions. As the fine edge of the pointer attached to the sights passed among those numerous divisions it touched one of them, suppose the fifteenth division on the sixth scale, then the angle was if of 90 - 15 52 56". This was a laborious method ; Tycho Brahe tried it, but aban doned it in favour of the diagonal lines then in common use, and still found on all scales of equal parts. In 1545 Dr Pedro de Medina published The Art of Navigation at Valladolid, dedicated to Don Philippe, prince of Spain. This appears to be the first book ever published professedly on navigation. It was soon trans lated into French and Italian, and many years after into English by John Frampton. Though this pretentious work came out two years after the death of Copernicus, the astronomy is still that of Ptolemy. The general appearance of the chart of the Mediterranean, Atlantic, and part of the Pacific is in its favour, but examination shows it to be very incorrect. A scale of equal parts, near the centre of the chart, extends from the equator to what is intended to represent 75 of latitude ; by this scale London would be in 55 instead of 51|, Lisbon in 37^ instead of 38 42 . The equator is made to pass along the coast of Guinea, instead of being over four degrees farther south. The Gulf of Guinea extends 14 degrees too far east, and Mexico is much too far west. Though there are many vertical lines on the chart at unequal distances they do not represent meridians ; there is no indication of longi tude. A scale of 600 leagues is given (German leagues, fifteen to a degree). By this scale the distance between Lisbon and the city of Mexico is 1740 leagues, or 6960 miles ; by the vertical scale of degrees it would be about the same; whereas the actual distance is 4820 miles. Here two great wants become apparent, a knowledge of the actual length of any arc, and the means of represent ing the surface of the globe on flat paper. There is a table of the sun s declination to minutes; on June 12th and December llth (o.s.) it was 23 33 . The directions for finding the latitude by the pole star and pointers appear good. For general astronomical information the book is inferior to that of Gemma. In 1556 Martin Cortes published at Seville The Art of Navigation. He gives a good drawing of the cross-staff and astrolabe, also a table of the sun s declination for four years (the greatest being 23 33 ), and a calendar of saints days. The motions of the heavens are described accord ing to the notions then prevalent, the earth being viewed as fixed. He recommends the height of the pole being found frequently, as the estimated distance run was imperfect. He devised an instrument whereby to tell the hour, the direction of the ship s head, and where the sun would set. A very correct table is given of the distances between the meridians at every degree of latitude, whereby a seaman could easily reduce the difference of longitude to departure. In the rules for finding the latitude by the pole star, the star is supposed to be then 3 from the pole ; it is now (1883) 1 18 54". Martin Cortes attributes the tides entirely to the influence of the moon, and gives instruc tions for finding the time of high water at Cadiz, when by means of a card with the moon s age on it, revolving within a circle showing the hours and minutes, the time of high water at the place for which it was set would be indicated. He deplores the loss of the earl of Niebla and other valiant captains of Spain, before Gibraltar, in 1436, because the mariners kept no account of the tides. In this instance it was more probably the effect of current. There is a chapter upon the signs which prognosticate fair or foul weather, from Pliny and Aristotle ; another upon " shining exhalations," the " fire of St Elmo," and other old superstitions. Directions are given for making a compass similar to those then in common use, also for ascertaining and allowing for the variation. The east is here spoken of as the principal point, and marked by a cross, after that the true north.
