Unifilar Magnetometer. xix 



Unifilar Magnetometer and Observations of the Absolute Horizontal 

 Intensity of the Earth's Magnetism. 



18. In the beginning of April 1843 two small wooden houses were erected about 

 19 yards to the north of the Magnetic Observatory ; the larger of the houses contains 

 the unifilar magnetometer and the dip circle, and the smaller, which is 10 feet to 

 SSE. of the larger, contains a reading telescope for the magnetometer. 



The unifilar magnetometer rests on a strongly-braced wooden stand, which is 

 fixed by copper battens and plaster of paris to a stone slab, resting on a stone founda- 

 tion 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 horizontal 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 (0.) 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 con- 

 nected, 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 

 3£ 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 Simms ; the gradua- 

 tions 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 coinciding 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 division 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 verified usually before each observation. 



