x INTRODUCTION TO THE MAKERSTOUN OBSERVATIONS, 1847 To 1855. 
iron bands of the steps upon the instrument was = — 2°9 scale divisions. All the 
observations of declination between these dates have in consequence been cor- 
rected by + 2°9 scale divisions. 
1849, July 10¢5*. Torsion removed, circle reading changed from 341° to 
3433°. Comparisons made before and after seemed to show that no torsion had 
been taken out. 
Sept. 254 2". Torsion removed, circle turned from 3434° to 327%’. 
Sept. 28°. Thread wound up about half an inch, and allowed to hang with a 
brass bar until Oct. 04 22" (another magnet being used in the interval), when the 
circle reading (left) was 2813°. The height of the magnet above the marble slab 
was measured, and the mean of the upper and lower edges (mean of north and 
south ends) was found to be 2°175 inches. Comparisons made between the declino- 
meter and another magnet before and after change, gave the effect of change 
inl; ; 
Oct. 284 23". Torsion tried, circle not altered, torsion less than 2°. 
1850, July 254 5". Observed height of declination magnet above marble slab. 
Mean of upper and lower edges (mean of north and south ends) = 27155 inch. In 
Oct. 04, 1849, it was 2°175 inch, so that the thread has only stretched 0-02 inch. 
Torsion circle, left reading B 307°; previously 288°. 
1854, Oct. 10%. Torsion removed, circle turned from 307° to 262°. 
BIFILAR OR HorIzoNTAL ForcE MAGNETOMETER, 
A detailed description of this instrument, of the method of observing it, and 
of reducing the observations, will be found in the Introduction 1845-46, p. xxv. 
It is not the object of the instrument to furnish absolute values of the hori- 
zontal component of the earth’s magnetic force, but merely to measure the varia- 
tions of this element. To accomplish this, a magnet is forced into a position at 
right angles to the magnetic meridian, and is there kept in equilibrium, by two 
forces, viz., the horizontal component of the earth’s magnetic intensity acting on 
the free magnetism of the bar, and the centre of gravity of the bar itself en- 
deavouring to attain the lowest position (see Introduction 1845-46, Art. 29). 
Let us suppose that the magnet has been forced into this position. Were 
the horizontal component of the earth’s magnetism, and the free magnetism of the 
bar both invariable, the position of the magnet would remain unaltered. 
But both of these being variable quantities, there are consequently two inde- 
pendent circumstances which may cause a change in the position of the magnet. 
1°, An increase of temperature, which would diminish the magnetic power 
of the bar, or a diminution of temperature, which would increase it. 
2°, Or a change may take place in the horizontal component of the earth’s 
magnetic intensity. 
As it is this latter change which we wish to measure, the effect of tempera- 

