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, 33 inches broad, and 1? 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 Sms ; 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 observation. 
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 w be the corresponding angle through which the suspended magnet is 
deflected, then 
be ais tan u 1 ae 
x 2 1+ Bie + &e. 
ped r 
where m is the magnetic moment of the deflecting bar, X the absolute horizontal 
