OF MAGNETIC DECLINATION. 353 



The angle 4. can be ascertained with the utmost accuracy by direct observa- 

 tion, for it is the supplement of the zenith distance of the collimator-cross, as 

 seen through the theodolite-telescope : but the ordinary methods of observation 

 do not afford sufficient data for ascertaining the angles a and /?. I have found, 

 however, that these angles may be computed, provided we observe the magnet 

 not only erect and inverted, that is when turned round its axis 0° and 180°, but 

 also when turned round 90° and 270°. 



8. In order to put this mode of observation in practice, I had a small colli- 

 mating magnet constructed by Mr Adie, shown in fig. 3, consisting of a hollow 

 steel cylinder 2*1 inches long, 05 inches in diameter, and about 0*04 inches thick, 

 furnished at the end N with an achromatic lens of about 2 inches focal distance, 

 and 0*3 inches aperture, and at C with a diaphragm carrying a cross of fine 

 spider-lines. The diaphragm is supported by four screws, s, having their heads 

 so deeply sunk below the surface of the cylinder as to be out of risk of dis- 

 turbance ; and the cell of the lens is made to screw into the cylinder, in order 

 to adjust the cross-wires to its principal focus. The cylinder having been placed 

 in temporary Ys, the lens was screwed in, until the wires were distinctly visible 

 through the theodolite-telescope, which had previously been brought to sidereal 

 focus ; and then, by means of the screws s, the diaphragm was adjusted until 

 its cross- wires continued nearly to intersect those of the theodolite-telescope, 

 while the cylinder was turned round in the Ys. In this manner, the line of col- 

 limation was rendered nearly parallel to the axis of the cylinder, and thus also 

 approximately parallel to the magnetic axis. 



9. On the cylinder there is a tightly fitting brass ring A, having on its sur- 

 face four lines, two of which, marked 1, 4, are shown in the figure ; and the cy- 

 linder itself turns without much friction in the ring B, to which the torsion - 

 fibre of silk, F, by which it is suspended, is attached. The lines on the ring A 

 are situated 90° from each other, and are marked for reference 1, 2, 3, 4 ; and 

 the ring B having also a line marked upon it, with which the lines on A may 

 be made successively to coincide, the cylinder may be suspended, either in its 

 usual position, or turned through 90°, 180°, or 270°. The ring A was turned on 

 the cylinder, until, when the lines 1 and coincided, the wires of the collimator 

 were sensibly horizontal and vertical, — a condition which was known to be ful- 

 filled when, on looking through the theodolite-telescope, it was possible, by means 

 of the tangent-screws, to make the vertical and horizontal wires of the theodolite 

 cover those of the collimator. 



In figures 4, 5, and 6, AB, CD are the wires of the theodolite-telescope, and ef, 

 gh those of the magnet-collimator, represented, for the sake of simplicity, as they 

 would appear when viewed through an erecting eye-piece. In figure 4, the mag- 

 net is in its usual or erect position, which is known to be the case when a small 

 index i, projecting from the edge of the diaphragm, is seen uppermost. In fig. 5, 



VOL. XXI. PART II. 5 c 



