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 300th division ; increasing readings of 
the scale indicate increasing magnetic force ; the axle of a grooved wheel ¢ 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 2, 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 verniersk. A 
copper ring encircles the magnet, in order to check the vibrations. A thermometer 
by Apr 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 1° 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. 3146" 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. 
