Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/263

NAVIGATION 251 the latitude by the pole star and the "pointers." There was not till 1607 any means whatever of measuring a ship s progress through the water, and none in general use till twenty or thirty years later (see LOG). The " cross-staff " appears to have been used by astro nomers at a very early period for measuring heights and distances, more recently by seamen for measuring alti tudes. It was one of the few instruments possessed by Columbus and Vasco da Gama. The old cross-staff, called by the Spaniards " ballestilla," consisted of two light battens. The part we may call the staff was about an inch and a half square and 36 inches long. The cross was made to fit closely and to slide upon the staff at right Fig. 1. angles; its length was a little over 26 inches, so as to allow the "pinules" or sights to be placed exactly 26 inches apart. A sight was also fixed on the end of the staff for the eye to peep through at the other two sights and objects to be measured. It was made by describing the angles on a table, and laying the staff upon it (fig. 1). The scale of degrees was marked on the upper face. After wards shorter crosses were introduced, so that smaller angles could be taken by the same instrument. These angles were marked on the sides of the staff. Another primitive instrument in common use at the beginning of the 16th century was the astrolabe, which was more convenient than the cross-staff for taking altitudes, but was incapable of measuring distances. Fig. 2 represents an astrolabe as described by Martin Cortes. It was made of copper or tin, about one-fourth of an inch in thickness and 6 or 7 inches in diameter, and was per fectly circular except at one place, where a projection was provided for a hole by which it was suspended. Weight was considered desirable in order to keep it steady when in use. The face of the metal having been well polished, a plumb line from the point of suspension marked the vertical line, which when carefully subdivided gave the horizontal line and centre. The upper left quadrant was divided into degrees. The second part was a pointer pt of the same metal and same thickness as the circular Fig- 2. plate, about 1^ inches wide, and in length equal to the diameter of the circle. The centre was bored, and a line was drawn across it the full length, which was called the line of confidence. On the ends of that line were fixed plates s, s, having each a larger and a smaller hole, both exactly over the line of confidence, as sights for the sun or stars. The pointer moved upon a centre the size of a goose quill. "When the instrument was suspended the pointer was directed by hand to the object, and the angle read on the one quadrant only. Some years later the other quadrant was also graduated, to give the benefit of a second reading. Among the earliest writers who touched upon naviga tion was John Werner of Nuremberg, who in 1514, in his notes upon Ptolemy s geography, describes the cross-staff as a very ancient instrument, but says that it was only then beginning to be introduced among seamen. He recommends measuring the distance between the moon and a star as a means of ascertaining the longitude. Thirty-eight years after the discovery of America, when long voyages had become comparatively common, Gemma Frisius wrote upon astronomy and cosmogony, with the use of the globes. His book comprehended much valu able information to mariners of that day, and was trans lated into French fifty years later (1582) by Claude de Bossiere. The system adopted is that of Ptolemy. The following are some of the points of interest for the subject before us. There is a good description of the sphere and its circles ; the obliquity of the ecliptic is given as 23 30 . The distance between the meridians is to be measured on the equator, allowing 15 to an hoar of time ; longitude is to be found by eclipses of the moon and con junctions, and reckoned from the Fortunate Islands (Azores). Latitude should be measured from the equator, not from the ecliptic, "as Clarean says." The use of globes is very thoroughly and correctly explained. The scale for measuring distances was placed on the equator, and 15 German leagues, or 60 Italian leagues, were to be considered equal to one degree. The Italian league was 8 stadia, or 1000 paces, therefore the degree is taken much too small. We are told that, on plane charts, mariners drew lines from various centres (i.e., compass courses), which were very useful since the virtue of the loadstone had become known ; it must be remembered that parallel rulers were unknown. Such a confusion of lines has been continued upon sea charts till very recently. Frisius gives rales for finding the course and distance correctly, except that he treats difference of longitude as departure. For instance, if the difference of latitude and difference of longitude are equal, the course prescribed is between the two principal winds, that is, 45. He points out that the courses thus followed are not straight lines, but curved, because they do not follow the great circle, and that distances could be more correctly measured on the globe. The tide is said to rise with the moon, high water being when it is on the meridian and nadir. From a table of latitudes and longitudes a few examples are here selected, by which it appears that even the latitude was much in error. The figures in brackets represent the positions according to modern tables, counting the longitude from the western extremity of St Michael. Flores is 5 8 farther west. Alexandria 31 Athens 37 Babylon 35 Dantzic 54 London 52 Malta 34 Kome 41 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 has 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 is here given (fig. 3). The description must necessarily be brief. The outer and principal sustaining circle EPQir represents the meridian, and is about 6 inches in diameter ; P, it are the poles. The upper quadrant is divided into degrees. It is suspended by K (31 13 ) 15 (37 58) (32 32) 30 (54 21) 3 (51 31) (35 43) 50 (41 54) 60 30 E. (55 55 ) 52 45 (49 46) 79 (70 25) 44 15 (44 38) 19 15 (25 54) 38 45 (40 31) 36 20 (38 30)