INTRODUCTION. XI 



Its sides are lined with velvet for more effectual exclusion of air-currents, and are removable for 

 manipulation of the magnet. 



A glass tube eight inches in length, pcrewed into the top of the box, carries a torsion circle 

 eight-tenths of nn inch in diameter, and divided to single degrees. A suspension pin four 

 inrhi'H long, and three-tenths of an inch in diameter, slides through a socket inthe centre of the 

 torsion circle, within which it is prevented from turning by a projecting pin fitting in a groove 

 cut down its whole length. The suspension pin may be held at any elevation by a pinching 

 screw on the upper part of the torsion-circle socket, and the fibre to sustain the magnet is at- 

 tached at an inverted triangular hole cut in its lower extremity. 



The lower suspension pin three-tenths of an inch long, and one-tenth in diameter has a 

 similar hole at its top. It turns freely within a small inilled-head cylinder, having a female 

 screw, by which it may be secured to the stirrup without torsion of the thread. The stirrup 

 consists of a pair of V supports at the extremities of a brass bar one and eight-tentha inches 

 long, and having a screw at the centre to fit the one just mentioned. 



The magnet is a hollow cylinder four inches long, and sixty-five hundredths of an inch but- 

 side diameter, with an achromatic lens at its north, and a divided glass scale in the opposite end. 

 It is fitted with a light and sliding brass ring to counteract changes of inclination and preserve 

 it horizontal, and two permanent bands to insure uniform position within the stirrup. A con- 

 cave mirror attached to the back of the box, and movable in all directions, serves to reflect light 

 through the collimating magnet. 



For detorsion of the suspension thread, the instrument has a brass plummet and a cylinder 

 of the same weight as the magnet, also fitted with a scale and lens, though the last is not achro- 

 matic. There is a weak magnet within the brass cylinder. To check the vibrations of the mag- 

 net, a copper damper, with open sides, accompanies each instrument, to be placed within the 

 wooden box. This has apertures in its ends, of one and two-tenths inches diameter, and a space 

 is cut out from the top that it may not disturb the suspension apparatus. 



A portable stand is furnished with the declinometer. This has a table top nine inches square, 

 with grooves for the foot-screws of the box, and which may be moved in azimuth or clamped 

 firmly by a capstan-headed screw underneath the centre. A spare tube, a reel of unspun silk, 

 and adjusting tools, also came with it. 



ADJUSTMENTS AND OBSERVATIONS. 



If the lens and scale of the collimating magnet are accurately adjusted by the maker, th 

 divisions of the latter are most distinctly seen at infini-distance focus, and it matters not at 

 what distance the alt-azimuth instrument is placed. Then the angular values of the divisions may 

 be determined by fixing the magnet, and measuring with the azimuth circle the horizontal - 

 angle subtended by any number of the scale intervals. But this accuracy of construction is 

 not always observed by the artist ; the scale may not be precisely in the focus of the lens, and 

 consequently, the angle subtended varies with the distance of the instrument by which it is 

 measured. Such defect in the magnet supplied the Astronomical Expedition was not suspected 

 until our return home, when it was ascertained that at two feet the angular value was 2' 14", 

 and at four feet was only 2' 7". And thus, as the distance between the declinometer and alt- 

 azimuth instrument was never measured at the time of observation, all the determinations of 

 absolute declination are doubtful to amounts whose maxima cannot exceed 25". 



To determine the division of the scale corresponding with the magnetic axis of the collimatDr 

 magnet, the telescope of the alt-azimuth instrument was directed to its central division, and 

 repeated readings were made in direct and inverted positions of the magnet. There being no 

 subsidiary apparatus convenient by which to ascertain changes of declination occurring during 



