a Knowledge of the Stars. 375 



about 140» : viz. 90° of north, and 40° of south declination, 

 which may be made to revolve upon the pillar as an axis inde- 

 pendently of the cone. To this part is attached the scale, L, 

 divided into degrees of north and south declination, corre- 

 sponding to those on the semicircle, — and so contrived as in 

 every situation to touch lightly the surface of the cone. 



To the declination circle K is attached the alidade M, which 

 may be set to any degree required, and serves as an index to 

 direct the eye of the observer to any object, which may be 

 viewed along the edge of it, or through the small holes in the 

 sights O P. 



Having described the several parts of the instrument, it only 

 remains to shew the use of it ; which will be best explained by 

 means of a few problems. 



Problem 1. To adjust the instrument. 



The instrument being made for the place of observation, need 

 only to be placed on a perfectly horizontal stand, and with the 

 line joining the hours XII and XII on the circle, in the direction 

 of the meridian : the former of these adjustments may be veri- 

 fied by means of a small level applied to the stand at N, in the 

 directions B C and A N successively. If found incorrect in 

 either position, let a piece of card be put under that foot 

 (A, B, or C) from which the bubble is found to recede, and 

 let this operation be repeated until the bubble rests in the 

 middle, in both positions. 



The instrument may be brought into the meridian by the 

 assistance of a magnetic needle fixed to the frame at N, or on 

 the opposite side, or more correctly by means of the sun, pro- 

 vided the time be exactly known, thus : — 



Set the index M to the sun''s declination, turn the circle K 

 round its axis, till the scale L points to the hour and minute 

 on the circle E F G. Then if the instrument be correctly 

 placed, the sun will be seen through the sights O, P, or what is 

 the same thing, the light admitted at the hole O will fall on 

 the hole P. If not, the instrument must be turned about till 

 this effect is produced. 



The instrument being once carefully adjusted to the meri- 

 dian on any immoveable stand, such as the sill of a window, 



