xxvi Introduction to the Makerstoun Observations, 1845 and 1846. 



at the extremity next the reading telescope, and a glass scale at the other : the scale 

 has 280 divisions, and the graduation at the SOOtli division ; increasing readings of 



the scale indicate increasing magnetic force ; the axle of a grooved wheel e fits into 

 the suspension eyes of the stirrup 6 ; the magnet, with these appendages, is borne 

 by a silver wire /, passing round the grooved wheel, and having its two extremities 

 pegged into a suspension roller g : the roller is supported by the torsion circle h, 

 which also bears, beneath the roller, a micrometer-headed screw ^, right-handed where 

 it meets one extremity of the wire, left-handed where it meets the other. The screw 

 serves to render the distance of the extremities of the wire equal to the diameter of 

 the grooved wheel ; the screw and suspension roller turn with the verniers k. A 

 copper ring encircles the magnet, in order to check the vibrations. A thermometer 

 by Adib and Son is enclosed by a glass tube passing through both boxes, the stem 

 of the thermometer, with the graduations, being above the lid of the outer box ; the 

 bulb of the thermometer rests in a cup, formed in a brass bar of the same dimensions 

 as the bifilar magnet ; the brass bar is supported on a wooden stand, and lies parallel 

 to the magnet ; the bulb of the thermometer is also covered loosely by a small brass 

 cap. It was found from comparative observations (p. xxx., Introduction, 1843) that 

 a thermometer, with its bulb free, would differ V from the thermometer resting in 

 the brass bar in the course of a daily change of 10° of temperature. The whole in- 

 strument was covered with a double thick cotton cover Jan. 31^ 6'' 1844. The read- 

 ing telescope is fixed to a stone pier, 8 feet south of the magnet. 



29. In the adjustment of the instrument, the magnet is forced to a position at 

 right angles to the magnetic meridian, by turning the arms of the torsion circle. 

 As, in forcing the magnet from the meridian, the upper extremities of the wire will 

 move through a greater angle than the lower extremities, the wires will be no longer 

 vertical, and the magnet and appendages will be raised ; the forces producing equi- 

 librium will, therefore, be the weight suspended endeavouring to attain the lowest 

 point, and the horizontal component of the earth's magnetic intensity acting on the 

 free magnetism of the bar. 



