Q64r Capt. Graydon's Celestial Compass, 



turned so as to bring the arm in a line between the axis P and 

 the sun S. The arm will then, by its motion, continue to 

 perform the same horary angle as the sun appears to describe, 

 and A P, being the noon hour line, will therefore always indi- 

 cate the true meridian. 



In like manner, the true meridian may be indicated by means 

 of a star, by calculating its distance from the plane of the me- 

 ridian, or its horary angle at the time of observation, and 

 setting the arm accordingly. 



As the relative position of the sun or a star, with regard to 

 the plane of the meridian, may be determined as above, its 

 declination may also be determined as follows: — (See fig. 5, 

 Plate 11.) 



Let fig. 5, Plate II. represent the exact position of the 

 sphere, having its axis P P elevated to the latitude of the place, 

 and the arm O o set according to the apparent time, — suppose 

 three hours, or 45° from the meridian, — and directed to the 

 sun S. 



The meridian distance of the sun S, or the angular distance 

 between the plane of the arm O o in that position, and the 

 noon hour line A P, will be equal to forty-five degrees ; be- 

 cause the instrument, being an exact representation of the po- 

 sition of the earth, having the axis P parallel to the pole, and 

 the noon hour line A P parallel to the plane of the meridian, 

 the horary angle is the measure of the meridian distance of the 

 object S. In like manner, a line E Q, which is perpendicular 

 to the axis P P, or to the pole, will be parallel to the plane of 

 the equator, with which it will continue to maintain the same 

 relative position during the motion of the arm O o round its 

 axis P P. 



As the arm Oso is an arc of a circle of which P is the cen- 

 tre, therefore, when the object has no declination, it will ap- 

 pear in the direction EQ, or the line SOP will coincide with 

 the line E P Q. But when the object S declines from the 

 equinoctial, the measure of its declination will be shown by 

 the part of the arc Oso which is above E Q, of which arc the 

 sight at P is the centre. The curve O s having a sight at O, 

 is moveable within the fixed part of the arm o, and may be 

 raised or lowered so as to bring the sight at O in a line be- 

 tween the object S and the other sight at P. 



The fixed part of the arm at o is graduated from each side 

 of ; the vernier upon the moveable curve at o will indicate the 

 amount of the angle of elevation or depression of the sight O, 

 above or below E Q, which angle is equal to the apparent de- 

 clination of the object S, at the time of observation. For ex- 

 ample, in the figure, the sight O is ten degrees above the equi- 

 noctial 



