Capt. Graydon's Celestial Compass. 359 



plate is screwed down upon the upper part of a box or case, 

 C, of a hemispherical form. The hemisphere C is suspended 

 upon pivots or axes at c, which work freely through holes 

 formed in a metal ring D ; and this ring is suspended upon 

 pivots or axes at d., being situated at right angles with the pi- 

 vots c, before mentioned. The pivots d are adapted to turn 

 in holes, or sockets, formed in adjustable bearings at the upper 

 part of the two standards, or supports, E E ; and the feet of 

 the standards are screwed down upon a plate of rhetal, F G, 

 which is capable of revolving upon an axis in the centre 

 thereof, affixed to the stationary platform, or board, H I. 



The plate F G has the cardinal points marked upon it, and 

 is divided into degrees near its outer edge, being provided 

 with a vernier I, affixed to the platform before mentioned. 

 K L represents a heavy plate of metal, which is suspended 

 from the pivots or axes c by two brackets, or open arms, one 

 only of which arms is seen in the figure at M, the other being 

 obscured by the hemisphere C. 



The plate K L is situated considerably below the centre of 

 gravity, or axis upon which the hemisphere C and ring are 

 suspended, and thus tends always to maintain the instrument 

 in a plane parallel to the horizon. 



The plate K L has an arm or limb rising up from it at K, 

 the upper part of which is provided with a vernier k, adapted 

 to read off the degrees upon a graduated arc, g h, engraved 

 on tho side of the hemisphere C. 



The under surface of the plate K L is provided with two 

 plane mirrors, or reflecting surfaces, m m, which are situated 

 in a frame affixed perpendicular to the plate K L, but in such 

 a position, that the reflecting surAices form a salient angle 

 with each othei\ 



These mirrors*, by reflecting the horizon from two diffe- 

 rent parts, furnish the means of adjusting the instrument into 

 a horizontal position when in use. For example : When any 

 two parts of the horizon are reflected in the mirrors vi m, if 

 the instrument be moved until the images of these two parts 

 api)ear upon the mirrors in one straight line, that line being at 

 the same time parallel to the edges of the mirrors, will indi- 

 cate that the plate K L, to which they are affixed, is hori- 

 zontal f . 



The pivots, or axes, upon which the hemisphere is suspended, 

 project some distance through the ring D, and are furnished 



♦ See fiR. 7, Plate II.; also see the end of y\ppentlix No. I. 

 f Anotiier metliod of adjusting the instrument to a horizontal position, 

 when the horizon is obscured, is given at the end of Appendix No. I. 



with 



