770 GEOGRAPHICAL LATITUDE. 



quadrant.^ The great English astronomer, Hadley, worked a revolu- 

 tion in observations of latitude at sea by publishing in 1731 the de- 

 scription of the octant,^ now known by his name," though it was proba- 

 bly invented by Newton.* By means of flat-surface reflectors the ob- 

 server is enabled to see apparently in the same line the two objects 

 whose angular distance he wishes to measure. The angle between the 

 two objects observed is measured by double the angle formed by the two 

 reflectors of the instrument, accordiDg to a well known law of reflected 

 light, which may be expressed as follows : " The angle between the first 

 and last direction of a ray which has suffered two reflections in the 

 same plane, is equal to double the angles formed by the reflecting sur- 

 faces."^ During most of the eighteenth century, the English not only 

 made great progress in astronomy, but were decidedly the leading mech- 

 anicians,^ so that their instruments were greatly sought for, even in 

 foreign countries. The necessity of having large instruments for ob- 

 serving latitude came to an end with the invention of Borda's circles 

 in 1790.' This instrument was not only made with extreme care and 

 with very minute divisions, but its form permitted a continuous series 

 of observations, each angle commencing where the preceding ended, 

 instead of the instrument being turned back to the starting point for 

 each new observation, as was the case with the sectors.^ In the pres- 

 ent century there have been constant improvements and alterations in 

 the forms and accuracy of observing instruments, a closer considera- 

 tion of which lies outside the scope of this article. The latest improv- 

 ment in principle seems to have been the invention of a reflection cir- 

 cle as a substitute for the sextant, which is provided with two nonii, 



iDelambre, Astron. dii xviii'""*, si^cle, p. 618, note. 



^The instrument in reality was only an octant, or formed an arc of 45°, but since, 

 through the principle of reflection, it could measure 90°, it is often called Hadley's 

 quadrant. 



='Delambre, Astron. dti xviii™'^, siecle, p. 688. 



* "He (Newton) also iuveuted a reflecting sextant for observing the distance be- 

 tween the moon and the fixed stars, the same in every essential as the instrument 

 which is still in everyday use at sea under the name of Hadley's quadrant. This 

 discovery was communicated by him to Dr. Halley in 1700, but was not published or 

 communicated to the Royal Society till after Newton's death, when a description of 

 it was found among his papers."— Encyc. Brit., art. "Navigation." 



^Herschel, Pop. Astron., §157, p. 122. 



eZach.Mon. Cor., 1804, p. 277. 



■> md.,j).i>71. 



«Delambre(Basedu systeme m^trique,!, 97,98),thu3speak8of the possible accuracy 

 of this instrument: "Lesdeux miens [instruments] 6taient divis^s en quatre cents 

 degr^s subdivises chacun en dix parties; ce qui faisoit en total quatre mille divisions 

 trac6es sur le limbe. Le vernier les partagoit encore chacune en dix parties, sans la 

 moindre incertitude, et I'oii pouvoit memeestimer, sans se tromper de deux en trois, 

 les millibmes de degr^. Quatre alidades, plac^es presque a angles droits, divisoient 

 encore I'erreur ; en sorte que ce n'est pas trop de dire que I'instrument donnoit les 

 milliemesde degr6. Ainsi, faisant abstraction des erreurs de la division, on auroit 

 uu angle h trois on quatre secondes pres, par une seule observation." 



