2 MAGNETIC A L OBSERVATIONS. 



containing a very weak magnetic needle, and the torsion-circle turned until the 

 collimater stood parallel to the box. Then the magnet was put in the place of the 

 collimater and adjusted in height and illumination. It was of cylindrical form and 

 hollow, the north-end being closed by a lens, and the south-end by a plane-glass 

 into which a fine scale was cut. 



The theodolite was mounted about one foot to the northward of the declinometer, 

 and so adjusted that the scale on the magnet appeared distinctly. The vertical wire 

 was put near the centre division, and after a set (generally three) of readings had 

 been made, the magnet was turned 180 around its axis, so that the numbers of the 

 scale, if they had appeared upright before, were now inverted. The first position 

 of the scale is always marked D. (direct), the second R. (reversed). This operation 

 was several times repeated, and the scale-division, corresponding to the axis of the 

 magnet, determined by combining the means of the 1st and 3d set with the mean 

 of the middle set, Next, the angle between the axis of the magnet and the mark 

 was measured by means of the theodolite ; and thus the magnetic azimuth of the 

 mark was determined. By these observations the telescope of the theodolite was 

 always several times reversed, so as to eliminate the error which might arise from 

 a deviation of the optical axis, which changes with the focal length. 



The declinometer was also used for observing the vibrations in the determination 

 of horizontal intensity. For this purpose two wooden arms were attached to it, 

 one carrying a telescope and ivory scale, the other a counterpoise-weight. After 

 levelling the instrument and taking the torsion out of the supension-thrcads by means 

 of the plummet, the magnet D 3 was suspended. It was of cylindrical form and 

 3.87 inches in length, and carries a small mirror (intended to reflect the ivory 

 scale), the plane of which was perpendicular to the axis of the magnet. A thermo 

 meter was placed in the box and read after each set of observations. The magnet 

 was steadied and the instrument turned in azimuth until the vertical wire of the 

 telescope covered the centre division of the reflected scale. The magnet was found 

 to hang so nearly horizontally, that it was not necessary to use a balance-ring. All 

 vertical motion was carefully checked and the magnet deflected from 100 to 180 

 scale-divisions from the meridian by means of another small magnet, and then left 

 to vibrate. The time of a passage of the central-division over the wire of the 

 telescope was now observed, and recorded as the time of commencement or vibra 

 tion, and afterwards the time, when the centre-division passed the wire again at 

 the completion of the l()th, 20th, 3()th, 40th, and 50th vibrations, was accurately 

 noted. The 100th, 200th, and 300th vibrations were also noted, and generally the 

 extreme scale readings were recorded at the same time. After the 300th again 

 every 10th to the 350th vibration was observed, thus obtaining six values for the 

 time, in which the magnet made 300 vibrations, viz., from to 300, from 10 to 

 310, etc. 



Occasionally the coefficient of torsion was determined by reading at first the 

 torsion-circle and the scale, then turning the torsion-circle 90 and reading the 

 scale again, then turning it 180 in the opposite direction, and finally turning it 

 back 90 to its original position, and reading the scale each time. It remained 

 always nearly the same and was very small. 



