NAVIGATING INSTRUMENTS. 



THE art of finding the latitude dates from early times. In 

 the process it is necessary to find the altitude of some 

 heavenly body whose position has been determined. The 

 first known instrument of any accuracy for this purpose was 

 the Astrolabe (Gr. astron = a star, tambano = to receive). 

 It was used by the Greeks and Arabians about B.C. 100, and 

 is said to have been first made by Hipparchus, who flourished 

 about 160-145 B.C. Chaucer, in his " Treatyse on the 

 Astrolabye," 1391, gives an excellent description of this 

 instrument, which consists of a heavy graduated metal circle 

 constructed to hang plumb from a ring at the top : " Thine 

 Astrolabye hath a ryng to putten on thy thombe on thy 

 righte honde in taking the heyght of thynges " (Chaucer). 

 It was still in use at the time of the Spanish Armada, speci- 

 mens having been found from the wrecked vessels, of which 

 there is an example among the collection of instruments at 

 the South Kensington Museum. Sir Jonas Moore, in his 

 " New Systeme of the Mathematicks," 1681, also describes it. 



Quadrants, with sights upon the side and a plummet hung 

 from the centre to pass the graduated arc and so to denote 

 the altitude, were also used. 



Fore Staff or Cross Staff. This instrument, according to 

 Seller in his " Practical Navigation " (edition 1711), was of 

 some antiquity, and at that date was probably much used, 

 as it is illustrated in most of the manuals of navigation of 

 that period. The staff was graduated either arithmetically, 

 by the table of natural tangents, or geometrically, from the 

 divisions of a quarter of a circle, and was fitted with crosses 

 or vanes, four in number, made to slide smoothly on the 

 staff. On one side of the staff the graduation began about 

 3 and proceeded to 10 toward the eye end, upon which 

 the 10 cross is used. The next is called the 30 cross and 

 belongs to the side of the staff where the graduations begin 

 at 10 and end at 30. The next is called the 60 cross and 

 works on the side where the graduation begins at 20 and 

 ends at 60; the longest is called the 90 cross for the last 

 side, where the divisions begin at 30 and end at 90. In 

 use the eye was placed at the end of the staff and the cross 

 moved until the top touched the sun or star and the other 

 end the horizon; the altitude was then read off from the 

 graduated side of the staff. (For article on the construction 



