ON THE KBW MAGNET06RAPHS. 331 



The coefficient, for converting the angular motions of the magnet into 

 variations of the whole vertical force, has been obtained by the usual method 

 of vibration in the horizontal and vertical planes. The magnet, with its ap- 

 pendages, having been first brought into approximate adjustment, was re- 

 moved and slung horizontally by a slight loop of thread attached to a silk 

 suspension. It was defended from currents of air by being enclosed in a 

 cylindrical box with lids, the thread bein"- alone exposed. A microscope 

 with cross wires was fixed to one side of this box, and so adjusted that a mark 

 on the shield carried by the magnet was visible. A vibration of about 2° 

 was given to the magnet, and the times of transit of the mark across the wire 

 noted by a chronometer. On March 31st the time of one oscillation was found 

 to be 17*90 seconds, the temperature being 52°. The time of vibration in 

 the vertical plane was observed by watching the motion of the image on 

 the ground-glass scale, the initial arc being generally nearly 2°. 



Owing to the arrangement of the stone pier upon which the apparatus 

 rests, and the position of the window from which light was obtained, the 

 magnet could not be conveniently mounted either in the magnetic meridian 

 or at right angles to it. It was in fact mounted at right angles to the astro- 

 nomical meridian, the north end being directed to about 67° west of the 

 magnetic north. The mode of adjusting the horizontality of the magnet was 

 the same as in Dr. Lloyd's original balance-magnets, namely, a screw attached 

 near the south end working horizontally. 



The adjustment for the height of the centre of gravity was effected at first 

 by altering the position of the weight which counterpoises the vertical arm 

 carrying the moveable shield. After August 8th this adjustment was made 

 by a screw working vertically in the same frame as the horizontal adjusting 

 screw, the former method having been found very inconvenient. 



In the beginning of August, the instrument, having been for some weeks 

 performing very indifferently, was returned to Mr. Barrow for alteration. 

 He stated that the knife-edges had become much deteriorated and even 

 somewhat rusted ; they were therefore re-ground : he was desired at the 

 same time to alter the mode of adjusting for the position of the centre of 

 gravity. The counterpoise weight below was permanently fixed; and a 

 screw working vertically attached to the south end, an equal weight being 

 added at the north end. On the magnet being returned it was again vibrated 

 horizontally ; and the time of one oscillation found to be 18*52 seconds, the 

 temperature being 72°. 



The temperature correction for this magnet was determined at the same 

 time as that for the horizontal force. The results were — At temperature 

 50°*4, the effect of one degree =0000283; at temperature 71°*5, the effect of 

 one degree=0*000319. 



The thermometer for this instrument was observed at the same time as 

 that of the horizontal force. The effect of the lamp in heating the air in 

 the magnet box was very trifling. 



Several circumstances have tended to prevent the satisfactory performance 

 of this instrument. When it was first put into action, the marble slab which 

 carries the magnet and its supports was suspended from the upper slab, upon 

 which were placed the camera, the lens, and the clock apparatus. This 

 upper slab again was merely laid upon the corbel supports, and not fastened 

 down to them. It was almost constantly noticed, during the month of April, 

 that the photographic trace exhibited sudden breaks or dislocations in the 

 curve, accompanied by oscillation of the magnet. These breaks could nearly 

 always be referred to periods when something was done in connexion with the 

 instrument, especially about sunset and sunrise, when the lamp was placed 



