280 



ARTIFICIAL HORIZON. 



to disturb its gravity,, or divert the si ghts.fr orpi their hoazon- 

 tal direction, when in use. 



If the ship have a considerable motion, it is advisable, to 

 suspend the box by a small brass giminal at the top to a por- 

 table stand; and to prevent the wind's affecting it, a glass 

 slides into each end of the box, through which the observa- 

 tion is taken; their surfaces being parallel, you adjust it by 

 the sea horizon, or by meridian altitudes of the sun, the la- 

 titude being known, or by any other method observers make 

 use of, or allow the index errour, as is done with the octants 

 and sextants. 



D is a small brass pin, to prevent the index from getting 

 any motion in carriage ; and is to be taken out, when the 

 horizon is in use. 



In observing the moon, planets, or stars, by night, a small 

 lanthoin with a lamp is necessary, to be placed behind the 

 box, so that the light may fall on the ivory, to show the line 

 distinctly ; and to prevent its spreading too much when you 

 are observing the stars, the glass is to be taken put, and the 

 brass with a small square hole, G, slid into its place. 

 Meiliod of To take an altitude with the octant and artificial horizon, 



taking an aiti- bring the eye as near to the horizon in the box as the frame 

 of the octant will admit of, and in a horizontal line look at 

 the tine edge of the sight E, which by the least motion of 

 the head you may bring into contact with the line on the 

 ivory sight F, and move forward the index on the frame of 

 the octant with your hand, to bring the object you are ob- 

 serving to a coincidence with the ivory line also. 



For altitudes of the sun or moon, and for all terrestrial 

 objects, an octant of the usual construction will answer 

 every purpose ; but for observing the stars, one with a larger 

 horizon glass, and its silvered surface also larger, with a dif- 

 ferent sight vane, would be preferable, and prevent your 

 inissing or mistaking a star when near to others, or its 

 getting out of the glass in bringing into contact with the 

 horizon. "With such a quadrant and this horizon, the me* 

 ridian altitudes of all bright stars, as they come to the me- 

 ridian, may be taken, by which means the latitude might be 

 frequently fouiid by ob .evvailons at night, and with as much 

 ease as by the sun at noon-day ; also, the altitudes of the 



moor* 



