10 
REPOET ON THE COMPLETION OF 
the threads thus constructed must have performed very indifferently ; for his obser- 
vations, which were made at the same time in the Paris Observatory, and in the 
caves below, shew differences and variations in the monthly means which are not 
explicable by anything- within our later, and, as we may suppose, more accurate 
experience. Since Cassini's time, the improvement of the suspension-thread 
seems to have made very little progress. M. Kupffer, apparently despairing 
of satisfactory results from a silk suspension, substituted silver wires in the 
Russian declinometers ; a similar suspension has also been employed by M. 
QuETELET at Brussels. This seems to me a step towards the cap and pivot sus- 
pension. Indeed, M. Nervander of Helsingfors has found that such suspension 
cannot be trusted, since the wires are so affected by temperature that, when an 
unmagnetic bar is suspended, it has a considerable diurnal motion : a fact which 
I had suspected, and had pointed out as a probable source of error in determining 
the temperature coefficient for the bifilar magnet. 
The suspension-thread acts in the following manner : — As the thread is com- 
posed of a series of fibres more or less twisted, the plane of detorsion, that is, the 
vertical plane in which an unmagnetic bar will rest when suspended, is deter- 
mined by the composition of series of opposing forces : if the torsion of the indi- 
vidual fibres be at all considerable, very small motions of the magnet will cause 
them to occupy slightly different positions inter se, or moderate changes of humi- 
dity acting to a greater extent upon the external than the internal fibres, and 
upon some of the external fibres than upon others, will change the plane of 
equilibrium, and in this way force the magnet from its true position. Changes of 
the plane of detorsion caused in this way, and by the occasional breaking of 
fibres, will explain the great discrepancies occurring in large series of observations, 
and the consequent lessening of their value. The importance also of obtaining 
properly-constructed suspension- threads will be at once evident, when it is remem- 
bered, that unless such are obtained, the labour of years may be rendered of little 
or no value. The conditions sought to be obtained in the construction of the thi'ead 
for the Makerstoun declinometer, were the following : — First, The absence of all 
torsion from the fibre. The so-called untwisted compound fibre from the cocoon, 
usually employed in observatories, receives a considerable twist in the operations 
of drawing from the cocoon and reeling, as may easily be perceived hy passing it 
between the thumb-nail and index, the method which I employed to remove the 
twist. Second, That each fibre when combined into the thread, shall beai" an equal 
portion of the weight. For this end the fibre w^as not cut into pieces, but a sufficient 
length being obtained, free of flaws, it was wound round two smooth pins, placed 
at the required distance, so that no twist should be introduced in the act of winding ; 
when a sufficient thickness was obtained, and the ends were tied, a hook attached 
to a Aveight was inserted in place of the lower pin ; the thread being formed of one 
continuous fibre was thus free to move round the upper pin and the weight-hook 
