IS 13.] on the Magnetic Variation. 97 



lines drawn upon them, which are brought to the points of the 

 needle when the observation is made. A light brass box, d d, 

 with a glass cover, is fitted over the plate C C, to preserve the 

 needle from disturbance from the wind ; it also supports a small 

 double microscope, M, intended to assist in examining when the 

 index line points exactly to the end of the needle ; the stem of 

 the microscope is fitted in a dovetail groove, and can be removed 

 to the opposite end of the box at pleasure. 



The centre pin of the plates E and C terminates in an ex- 

 tremely fine point, on which the needle is suspended, having an 

 agate cap, B, to diminish the friction as much as possible. The 

 needle is of a cylindrical figure, about the five-hundredth of an 

 inch in diameter, and ten inches long, and weighs 65i grains, 

 and is terminated by two conical points; the circular enlarge- 

 ment of the centre has a hole through it, for the reception of the 

 brass socket, B, which has the agate cap fixed in it ; the needle 

 is, of course, provided with the usual apparatus for lifting it off 

 the point when not in use, to avoid wearing the point of suspen- 

 sion, a, a, are two arms, screwed down upon the plate, carrying 

 the vernier, D ; they support a brass frame, b, which has the 

 usual clamp screw, L, and tangent screw, R, the former to fasten 

 it to the arch, m, and the latter to give a slow motion, and adjust 

 the box for the observation. 



The transit telescope, O P, is supported over the instrument 

 bygwo pillars, N, N, fixed on the brass plate, E E, and having 

 smalt frames or boxes, f, at the top, for the reception of the 

 Y's, in which the pivots of the axis, Q, of the telescope are 

 supported ; this axis is conical, and is fixed exactly at right 

 angles to the tube, O P, in the same manner as other transit 

 instruments. 



On the extremity of one of the pivots of the axis, a small 

 divided circle, K, is fixed, and has an arm at the opposite side 

 turning about on the centre, and provided with verniers to read 

 the divisions on the circle ; to this arm a small level, S, is 

 attached; the whole forming a flying index to set the telescope at 

 any required altitude, by setting the index at the proper divi- 

 sion, and then moving the telescope till the bubble of the level, 

 S, shows the index to be horizontal ; the eye-piece, h, of the 

 telescope has a small dovetail in it to admit a dark glass for obser- 

 vation of the sun ; g, are the screws for adjusting the wires in 

 the eye-piece as usual ; the separate figure, T V, is a level to 

 adjust the- instrument. 



Manner of ad listing the instrument previous to observation. — • 

 Fint place the feet, k, /, of the level, upon the brass plate, E E, 

 in different directions, and bring it level by the screws, II, I, K ; 

 then apply the level upon the two pivots of the tran-it, the covers 



Vol. II. N° II G 



