ASTRONOMY OF EIGHTEENTH CENTURY. 



253 



cult on account of the movement of a vessel, and others required the 

 observations to be reduced by too many corrections. Hadley's quad- 

 rant was, however, an invention which allowed the observations to be 

 accurately and easily made, and La Caille, Mayer, and Maskelyne 

 advocated the adoption of this method of observation in preference 

 to all others. And to obviate the necessity of laborious calculation by 

 the navigator, the British Board of Longitude undertook to publish 

 annually a Nautical Almanack, in which the distances of the moon 

 from certain stars, calculated for every three hours, were set down for 

 three years in advance. 



As it was the invention of reflecting instruments which made this 

 method of finding the longitude possible, it may be interesting to ex- 

 amine the construction of Hadley's instrument. The diagram in 

 Fig. 123 may be first referred to as 

 showing the principle of the instrument 

 without the details. A B is a graduated 

 arc, the centre of which is at c. This 

 arc may be one-quarter of the circum- 

 ference, when the instrument properly 

 takes the name of quadrant ; or it may 

 be one-eighth of the circumference, in 

 which form it is called the octant; or it 

 may be (and this is the most usual form) 

 one-sixth of the circle, and it is then 

 called the sextant. The close parallel 

 lines at c represent a flat mirror attached 

 to and turning with the arm c B, movable 

 about c, the centre. In the figure we 

 see only the edge of the mirror, and the 

 same at L, where the short sloping line 

 represents another mirror also perpen- 

 dicular to the plane of the arc, and per- 

 manently fixed to the frame of the instrument. It is in a peculiarity 

 of this mirror that the special power of the instrument resides. It is 

 made of glass, and only the half of it is silvered, the upper half being 

 left so that it does not interrupt the view of objects to which the tele- 

 scope/is directed. This telescope is fixed to the frame of the instru- 

 ment, and does not move with the arm c B. The silvered half of the 

 mirror L reflects to the eye of the observer at /rays which first fall 

 upon the mirror c. The mirror L is so fixed that it is parallel to mirror 

 c when the index of the movable arm c B points to o. This is accom- 

 plished by first fixing the movable arm at o, and then so adjusting the 

 mirror L, that when the sea-horizon is seen through the telescope, the 

 direct view H L/, and the reflected view, due to rays taking the path 

 K c L/, shall coincide. Now, suppose it is required to measure the 

 altitude of a star above the horizon : the instrument is held so that 



