TRANSACTIONS OF THE SECTIONS. 11 
formed several times between his fingers, which had been dipped in slightly gummed 
water: after leaying the thread with a weight suspended for twenty-four hours, 
he again passed the thread between his fingers, which were greased with tallow*. 
Cassini’s observations were made in the Paris Observatory and in the caves below; 
as far as can be judged from their monthly means, the threads seem to have per- 
formed yery indifferently. Since Cassini’s time the improvement of the suspension 
thread seems to have made very little progress; in general the thread has been 
‘formed by the combination of a series of fibres, or by a reduplication of the same 
fibre, without any preparation, and just as it has been found on the reel. M. Kupffer, 
apparently despairing of satisfactory results from a silk suspension, substituted 
silver wires in the Russian declinometers; a similar suspension has also been 
adopted by M. Quetelet at Brussels. This seems to me a step backwards. 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 sus- 
pended 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 
of the bifilar magnet. M. Nervander has proposed to form the suspension thread 
by moistening with hot water the fibre cut into lengths, and submitting each length 
in this state to a considerable tension before combining to form the thread. I formed 
the thread for the declinometer in Sir Thomas Brisbane’s Observatory at Makers- 
toun in the year 1843 in the following manner:—I had observed that the fibre, 
which is wound ona reel and termed untwisted silk, has in reality a considerable 
twist ; each fibre is not simple but compound, and the simple fibres are at first more 
or less twisted around each other, as may be easily understood when the operation 
of forming the compound fibre from the cocoon is considered; the further process 
of reeling also induces a considerable twist. I first, therefore, removed all twist 
from the compound fibre by running as much as would form twenty-two times the 
length of the required thread between the finger and thumb, and then wound the 
continuous fibre on two smooth pins placed at the requisite distance, so that no 
twist should be introduced in the act of winding ; after tying the extremities, a hook 
carrying a weight 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 till each length bore nearly an equal strain. After the weight had been sus- 
pended for some time, the fibres were tied firmly together at short intervals by small 
pieces of cotton thread. ‘This suspension has performed very well for seven years. 
Up to the present time, however, I am unacquainted with any comparative obserya- 
tions with differently constructed suspension threads. In the end of June I requested 
Mr. Hogg, an Assistant in Sir Thomas Brisbane’s Observatory, to construct, as 
carefully as possible, three threads ; one according to Cassini’s process, one by M. 
Nervander’s, and one by my own. Each thread was formed of the same number of 
lengths of fibre ; they were suspended in the same closed box with glass sides ; each 
carried a weight of nearly a pound, with a small index for reading the variations of 
the plane of detorsion. The torsion forces of the three threads were in the follow- 
‘ings ratios :— 
) Cassini : Neryander : Brown = 12:11: 6. 
From twenty-five days’ observations the mean changes of the plane of detorsion from 
day to day, independent of sign, were— [pee 
Cassini 2°°5 3 Nervander 2°1; Brown 25 ; 
or reducing to the same torsion force, the ratios of the variations of the plane of de- 
torsion were— 
Cassini = 30; Nervander = 23; Brown = 12. 
The differences of the variations of the plane of detorsion between 7 P.M. and4 p.m. 
are fully more marked ; when reduced to the same torsion force, the ratios were— 
Cassini = 46; Nervander = 37; Brown = 14. 
So that the thread prepared according to my own process is at least twice as good 
* Journal de Physique, t. xl. p. 344. 
4 
