MAGNETIC OBSERVATIONS. 39 



made by shifting it endwise in its stirrup till the scale was seen in the field of the 

 telescope parallel to, and just in contact with, the horizontal wire. 



When making my first observations considerable difficulty was experienced in 

 getting a proper illumination of the magnet scale, but after some practice the fol- 

 lowing perfectly satisfactory plan was adopted. In cloudy weather the light of a 

 white cloud was reflected into the magnet by means of the concave mirror. In 

 clear weather the light of the blue sky, reflected from the mirror, was not sufficient, 

 and it would not do to throw in the direct rays of the sun because of their heating 

 power, which would certainly have led to the use of a wrong value of the magnetic 

 moment; because the magnet would have been at a higher temperature than that 

 shown by the thermometer in the box. Under these circumstances, in place of the 

 mirror a piece of perfectly white paper was substituted, and the direct rays of the 

 sun being allowed to fall upon it, it afforded a beautiful illumination of the magnet 

 scale. 



The copper damper, provided to slip into the magnet-box for the purpose of 

 quieting the vibrations of the magnet, was never used. As the observations were 

 all made in the open air, and as there was frequently wind enough to cause the 

 instruments to vibrate perceptibly, the magnets seldom or never came to a state of 

 absolute rest. Hence, the plan adopted to secure accurate readings of the scales 

 was as follows. A screw-driver was slightly magnetized, and by approaching its 

 south pole for an instant towards the south pole of the vibrating magnet, at a time 

 when the magnet was moving towards the screw-driver, the arc of vibration was 

 readily made quite small. Then, placing my eye to the telescope, I read off, and 

 called out to my assistant, the scale reading at the instant the magnet attained the 

 limit of its excursion in the eastern direction, and again when it attained the limit 

 in a western direction — in other words, the greatest and least readings of the scale 

 were noted. Five complete vibrations were generally observed, thus giving three 

 eastern and three western readings, and the mean of the six was assumed to be 

 the reading which would have been obtained if the magnet had been in a state 

 of perfect rest. 



In order to preserve the magnetism of the collimator magnets, they were always 

 packed in a vertical position, with that pole downwards which would be lowest in 

 a dipping needle. 



Absolute Declinations were observed as follows: The instruments having been 

 set up and adjusted in the manner already explained, the long magnet, C. 32, was 

 suspended in the magnet-box, the telescope pointed nearly to its magnetic axis, and 

 the horizontal circle of the theodolite firmly clamped. Then, 1°. The horizontal 

 limb of the theodolite was read. 2°. The magnet scale being erect — that is, the 

 figures upon it being right side up — the point upon it cut by the vertical wire of 

 the telescope was observed. 3°. The telescope remaining as before, the magnet 

 scale was inverted — that is, the magnet was turned on its axis through 180°, so 

 that the figures upon its scale were seen inverted — and the point upon it cut by 

 the vertical wire was again noted. 4°. The horizontal circle was undamped, a 

 colored glass placed upon the eye-piece, and the telescope pointed so that its verti- 

 cal wire was just in advance of the first limb of the sun. Then the horizontal circle 



