SURVEYING INSTRUMENTS. 101 



levelling remaining undisturbed, and the plane will then be 

 horizontal. Price, $6.00 to $10.00. 



When an artificial horizon is used, the observer must 

 place himself at such a distance that he may see the re- 

 flected object as well as the real one; then, having the 

 sextant properly adjusted, the upper or lower limb of the 

 sun's image supposing (that the object) reflected from the 

 index-glass, must be brought into contact with the opposite 

 limb of the image reflected from the artificial horizon, ob- 

 serving that when the inverting telescope is used, the upper 

 limb will appear as the lower, and vice versa; the angle 

 shown on the instrument, when corrected for the index- 

 error, will be double the altitude of the sun's limb above 

 the horizontal plane ; to the half of which, if the semi- 

 diameter, refraction, and parallax be applied, the result 

 will be the true altitude of the centre. 



9* 



