INVESTIGATION 



ELEVEN (OE TEN) YEAR PERIOD, AND OF THE DISTURBANCES OF THE 



VERTICAL FORCE. 



THE observations of the vertical component of the magnetic force were com- 

 menced in June, 1840, and continued, with an exception in January, 1841, without 

 interruption to the last of June, 1845. To keep up the continuity of the series, a 

 daily reading was taken at 2 h 17 m P.M. during the months of January, February, 

 and March, 1843. Up to October, 1843, the observations were bi-hourly, after- 

 wards hourly. 



Instruments. From June, 1840, to the end of the year, the observations were 

 made with a balance vertical force magnetometer of Lloyd's pattern. It was at 

 first mounted in the eastern building of the College, but was removed to the obser- 

 vatory in the latter part of July. While in the College an increase of the readings 

 corresponds to a decrease of vertical force ; at the observatory increasing readings 

 denote increasing force. The instrument was made by Robinson, of London ; the 

 magnet, the axis of which was mounted as nearly as possible transversely to the 

 magnetic meridian, was 12 inches in length, having at its ends cross wires set in 

 copper rings. For a full description see Dr. Lloyd's account of the Magnetical 

 Observatory of Dublin, and the preface in volume I, of the record of the Phila- 

 delphia observations. 



In January, 1841, the Lloyd instrument was replaced by a reflecting vertical 

 force magnetometer, made at my suggestion by Mr. J. Saxton. The bar of this 

 instrument was two feet and one inch in length, two inches wide in the middle, 

 one and a half near the ends, tapering to nothing at the ends, and a quarter of an 

 inch thick. The magnet was of steel and hardened as perfectly as the maker could 

 effect. By means of a ball moving on a fine screw, its equilibrium could be changed. 

 The mirror projected outside the box, and the motion of the bar was observed by 

 means of a telescope. At the top of the box was a piece of plate glass through 

 which a thermometer (of Francis' make) could be read. For further particulars 

 see p. vii, of the preface to volume I of the record. For some time (between 

 three and four months) after being put up, the bar lost considerably of its magnetic 

 force, and after being in use four months, a movement of the adjusting ball upon 

 the screw was required for placing the readings again near the middle of the scale. 

 By this adjustment, the sensibility of the apparatus was not interfered with. 



The value of a scale division of. the Lloyd instrument, expressed in parts of the 

 vertical force, was carefully determined and found to be =0.0000165, both in the 



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