THE CONQUEST OF THE ZONES 



and having only half its surface mirrored) fixed in the 

 line of vision of a small telescope; the other (called 

 the index mirror) movable with the arm of an indicator, 

 which is so adjusted as to revolve about the axis of the 

 quadrant. In operation these two mirrors enable the 

 images of two objects, the distance between which is 

 to be measured, to be superimposed. The telescope 

 may be pointed at the horizon, for example, directly 

 under the position of the sun, and the arm of the in- 

 strument, altering the position of the so-called index 

 mirror, may be rotated until the Hmb of the sun seems 

 just to touch the horizon — the latter being viewed 

 through the unsilvered half of the horizon glass. The 

 scale at the circumference of the instrument is marked 

 in half-degrees, which, however, are registered as 

 whole degrees, and which, so interpreted, give the 

 direct measurement of the angular distance between 

 the horizon and the sun; in other words the measure- 

 ment of the sun's altitude or so-called declination. 



The instrument just described, perfected as to details 

 but not modified as to principles, constitutes the modem 

 sextant, which is used by every navigator, and which 

 constitutes, along with the compass and chronometer, 

 the practical instrumental equipment that enables the 

 seaman to determine — ^by using the tables of the 

 Nautical Almanac — his exact position on the earth's 

 surface from observation of the sun or certain of the 

 fixed stars. The modem instmment is called a sex- 

 int because it has, for convenience' sake, been re- 

 stricted in size to about one-sixth of a circle instead 

 of the original one-quarter, the small size being found 



[21] 



