Page:EB1911 - Volume 19.djvu/301

 represent the positions according to modern tables, counting the longitude from the western extremity of St Michael. (Flores is 5° 8′ farther west.) The latitude of Cape Clear is given 34′ in error and the longitude 4°; the Scilly Islands are given with an error of one degree in latitude and 1° 10′ in longitude; while Madeira is placed 3° 8′ too far south and 4° 20′ too far west, and Cape St Vincent 1° 25′ too far south and 6° too far west.

In 1534 Gemma produced an “astronomical ring,” which he dedicated to the secretary of the king of Hungary. He admitted that it was not entirely his own invention, but asserted that it could accomplish all that had been said of quadrants, cylinders and astrolabes—also that it was a pretty ornament, worthy of a prince. As it displayed great ingenuity, and was followed by many similar contrivances during two centuries, a sketch with brief description is here given (fig. 4).

The outer and principal sustaining circle EPQ represents the meridian, and is about 6 in. in diameter; P, are the poles. The upper quadrant is divided into degrees. It is suspended by 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 measuring heights. Its use is illustrated by twenty problems, showing it capable of doing 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 corresponds with 12 o’clock. Look 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 represented as 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 suspension 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.

There is in the museum attached to the Royal Naval College at Greenwich an instrument described as Sir Francis Drake’s astrolabe. It is not an astrolabe, but may be a combination of astronomical rings as invented by Gemma with additions, probably of a later date. It has the appearance of a large gold watch, about 2 in. in diameter, and contains several parts which fall back on hinges. One 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, and 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 3 m. too much.

Though clocks are mentioned in 1484 as recent inventions, watches were unknown till about 1530, when Gemma seized the idea of utilizing them for the purpose of ascertaining the difference of longitude between two places by a comparison between their local times at the same instant. They were too inaccurate, however, to be of practical use, and their advocate proposed to correct them by water-clocks or sand-clocks. For rough purposes of keeping time on board ship sand glasses were employed, and it is curious to note that hour and half-hour glasses were used for this purpose in the British Navy until 1839. The outer margin of the compass card was early divided into twenty-four equal parts numbered as hours until the error of thus determining time by the bearings of the sun was pointed out by Davis in 1607.

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 given is one for finding the latitude of a place by knowing the sun’s declination and altitude when on two bearings, not less than 40° apart. Gemma did a similar thing with two stars; therefore the problem now known as a “double altitude” is a very old one. It could be mechanically solved on a large 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, &c., the credit of which is due, however 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 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 of 90°＝15° 52′ 56″. This was a laborious method; Tycho Brahe tried it, but abandoned it in favour of the diagonal lines then in common use and still found on all scales of equal parts.

In 1545 Pedro de Medina published Arte de navigar at Valladolid, dedicated to Don Philippo, prince of Spain. This appears to be the first book ever published professedly entirely on navigation. It was soon translated 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 given 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° 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; and there is no indication of longitude. 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 representing the surface of the globe on flat paper. There is a table of the sun’s declination to minutes; on June 12th and December 11th (o.s.) it was given as 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 Arte de navigar. He gives a good drawing of the cross-staff and astrolabe, also a table of the sun’s declination for four years (the greatest value being 23° 33′), and a calendar of saints’ days. The motions of the heavens are described according to the notions then prevalent, the earth being considered as fixed. He recommends