THE CONQUEST OF TIME AND SPACE 



to answer every practical purpose, since it measured- 

 all angles up to 120 degrees. Q 



In practice the sextant is an instrument only six or 

 eight inches in diameter. It is held in the right hand 

 and the movable radial arm is adjusted with the left 

 hand with the aid of a micrometer screw, and the read- 

 ing of the scale is made accurate by the vernier arrange- 

 ment. The ordinary observation — which every traveler 

 has seen a navigator make from the ship's bridge just 

 at midday — is carried out by holding the sextant in a 

 vertical position directly in line of the sun, and sighting 

 the visible horizon line, meantime adjusting the re- 

 cording apparatus so as to keep the sun's limb seeming- 

 ly in touch with the horizon. As the sun is constantly 

 shifting its position the vernier must be constantly ad- 

 justed until the observation shows that the sun is at the 

 very highest point. The instrument being clamped 

 and the scale read, the latitude may be known when 

 proper correction has been made for the so-called dip, 

 for refraction, and where great accuracy is required 

 for parallax. 



Dip, it may be explained, is due to the fact that th 

 observation is made not from the surface of the wat 

 but from an elevation, which is greater or less acco 

 ing to the height of the bridge, and which therefore 

 varies with each individual ship. The error of refrac 

 tion is due to the refraction of the sun's light in passing 

 through the earth's atmosphere, and will vary with the 

 temperature and the degree of atmospheric humidity 

 both of which conditions must be taken into accoun 

 The amount of refractive error is very great if an objet 



[22] 



p 



