xxiv Introduction to the Makerstoun Observations, 1843. 



foundation separated from the floor ; the top block of the stand, a solid piece of 

 mahogany, carries a vertical box enclosing the suspension thread and supporting 

 the torsion circle, this box is open on two opposite sides near the stand top ; a hori- 

 zontal box slides on the vertical one, and when close to the stand top the magnet is 

 completely enclosed ; an internal box was afterwards added, and all the precautions 

 already indicated (6.) for the declinometer were taken. The magnet used when 

 observations of absolute horizontal intensity were made was that usually in the 

 declinometer, a spare magnet being fitted with a short scale (8.) was substituted for 

 it ; the telescope (that intended for a collimator to the bifilar) was placed in the 

 smaller wooden house, on a stand in all respects similar to that for the unifilar : the 

 two houses were connected, during observations, in the line of collimation of the 

 telescope and magnet by a wooden tube blackened within. A beam of straight well 

 seasoned fir, 11 feet long, 3f inches broad, and IJ inches thick, was placed on each 

 side (outside) of the larger wooden house, in the line passing through the centre of 

 the suspended magnet, and at right angles to the magnetic meridian ; each beam 

 was let into the tops of two strongly braced wooden trestles, 7 feet apart, which 

 rested on wooden posts driven into the ground, and which were fixed to the latter 

 by catch pins, allowing a slight adjustment for the distance of the beams from the 

 magnet ; the trestles and beams being removed after each observation. The beams 

 were carefully divided with the aid of a brass standard yard made by Messrs 

 Troughton and Simms ; the graduations were adjusted to their distance from the 

 suspended magnet in the following manner : — a well seasoned fir rod, shod with 

 brass at one extremity, and terminated at the other by a capstan-headed screw, by 

 which the rod was accurately adjusted to a length of six feet, was passed through 

 holes in the sides of the wooden house and unifilar box ; the middle of the rod coin- 

 ciding with the suspension thread, the catch pins of the trestles were then loosened 

 or forced in till the extremities of the six feet rod coincided accurately with the divi- 

 sion 3 feet on each beam. The deflecting magnet was adjusted to the graduations 

 on the beams with the aid of a lens ; in 1844 the graduations were marked on brass 

 pin heads placed in the beams. The fixidity of the trestles was verified in general 

 after each observation, and the accuracy of the graduations on the beams was veri- 

 fied usually before each obser\^ation, 



26. The value of the absolute horizontal intensity is determined from the ob- 

 servations as follows : — if r be the distance at which the centre of the deflecting 

 magnet is placed on the wooden beam (in the direction of the central line of the 

 beams), and u be the corresponding angle through which the suspended magnet is 

 deflected, then 



ml,, "" 



— = — r^ tan u 



where m is the magnetic moment of the deflecting bar, X the absolute horizontal 



