2 MAGNETICAL OBSERVATIONS. 



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

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

 coUimater and adjusted in heiglit 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 Avas 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 rcAerscd, 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 Avere attached to it, 

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

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

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

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

 scale), the plane of Avhicli Avas perpendicular to the axis of the magnet. A thermo- 

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

 Avas steadied and the instrument turned in azimuth until the vertical Avire of the 

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

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

 vertical motion Avas carefully cheeked 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 Avire of the 

 telescope Avas noAv observed, and recorded as the time of commencement or vibra- 

 tion, and afterAvards the time, when the centre-division passed the Avire again at 

 the completion of the 10th, 20th, 30tli, -lOth, and 50th vibrations, Avas accurately 

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

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

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

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

 310, etc. 



Occasionally the coefficient of torsion Avas 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 

 alAvays nearly the same and Avas very small. 



