POSTSCEIPT. Ixv 



116. The observations of both the force magnetometers have been corrected for 

 temperature, it is conceived, with a considerable approximation to accuracy ; but not 

 wishing to dogmatize in the use of a new mode of determining the temperature co- 

 efficient, I have, with Sir Thomas Beisbane's leave, printed in all cases the tem- 

 peratures of the magnets. In this, as in some other cases, I have preferred giving 

 what may seem at present too much, rather than any one should afterwards have 

 reason to find that I had given too little. 



117. All the reductions have been made by my present assistants, Messrs 

 Welsh and Hogg, and by myself. Each computation has been performed twice, 

 and that generally by different individuals. 



Makekstoun, June 1846. 



Postscript. 



Value of the Scale Divisions of the Bifilar Magnetometer in parts of the whole 



Horizontal Force. 



118. A consideration of the theory of the bifilar magnetometer will shew that it 

 is assumed that the suspending wires do not act at all by any elastic force ; that, in 

 fact, the force opposing the magnetic force is the resolved portion of that due to the 

 weight suspended endeavouring to gain its lowest point, and, therefore, that if u be 

 any angle from the magnetic meridian to which the magnet is deflected, the corre- 



spending torsion of the wires being v (No. 35.), then — — is a constant ratio. If 



the assumption fail, there will be every reason to doubt the accuracy of the coeffi- 

 cient h^ which depends on sin v and its difference. Any considerable error was not 

 suspected ; but the method described in the note, pages 2 and 3, having been found 

 to answer so well for the determination of the coefficient for the balance needle, there 

 was little doubt but that it would succeed much better for that of the bifilar magnet. 

 Experiments were accordingly made when the previous Introduction was nearly 

 through the press. 



119. If the equation of equilibrium for the bifilar magnet when at right angles 

 to the magnetic meridian be (No. 35.) 



»^ X=/ 



and if a magnet, whose moment is M, be placed in the magnetic meridian, with its 

 centre in the continuation of the bifilar magnet when at right angles to the magnetic 

 meridian, and at a distance r from its centre, the resulting angle of deflection being 

 A V, equal n scale divisions, the equation of equilibrium will be (see the note already 

 referred to), 



m I X + ■ — g — J cos A v^=/ 



MAG. AND MET. OBS. 1843. r 



