for ascertaining the Deviation of the Magtietic Needle. 363 



scribed, and the line F G is to be placed north and south, by 

 means of the magnetic compass in the binnacle*. The hemi- 

 sphere C is then to be inclined upon its axis c, and the hand E 

 moved, until the shadow of the intersection of the cross wires 

 O is brought to coincide with the intersection of the marks 

 upon the surface of the tablet q ; the vernier k will then indi- 

 cate the latitude of the place, by means of the divisions upon 

 the arc g h.\ 



In hke manner, the apparent time may be obtained when 

 the latitude is given, by setting the arc gh to the latitude, the 

 rod s to the sun's declination ; the hand E is then to be moved 

 until the shadow of the intersection falls upon the cross lines 

 upon the ivory q, as before described. The vernier at the end 

 of the hand E will then indicate the apparent time, even when 

 the horizon is obscured, and when an altitude cannot therefore 

 be taken J. 



Appendix. — No. I. 

 The following is the principle upon which the celestial com- 

 pass is constructed, viz. : 



The apparent motion of the sun being caused by the uni- 

 form rotation of the earth upon its axis from west to east once 

 in twenty-four hours, it will appear evident, that, if an arm be 

 moved with an equable motion in a contrary direction to the 

 apparent motion of the sun, and having its axis of motion in a 

 position parallel to the earth's axis §, the arm would keep pace 

 with the apparent motion of the sun, while it describes its 

 horary angle : if, therefore, the time be given, and the arm be 

 set to that time, and be directed towards the sun, the noon 

 hour line would then be situated in the true meridian. 

 For example, see fig. 4 and 5, Plate II. 

 Let O P represent the arm A F G, a circle described about 

 its axis P, and (hvided into twenty -four equal parts or hours. 

 When the arm O P is set to the apparent time, — suppose three 

 hours, or \:5° from the meridian,— the instrument is then to be 



• Of course the variation must be allowed for. 



t When the apparent time is given, the latitude may be determined 

 without using the magnetic compass, by setting the hand E to the time, 

 the rod i to the sun's declination, and then inclining the hemisphere C 

 until the sun's rays, passing through the lens at O, are concentrated upon 

 the middle of the tablet (j, as before mentioned. 



X As the method of adjusting the instrument to a horizontal position by 

 means of the reflecting mirrors m m, cannot be used when the horizon is 

 obscured, another mode of adjustment, by means of the sun's declination, 

 is given at the end of Appendix No. I. 



§ The earth, seen from the sun, would appear but as a point, and the 

 centre oi' motion of the arm may therefore be considered as coinciding with 

 - that of the earth. 



Z / 2 turned 



