520 



Major J. Herschel. 



angle. The double wires are in a state of torsion, but the single wire 

 hangs freely, helping by its tension to increase the tension on the 

 double wires caused by the major weight, but not itself in a state of 

 torsion. Now, suppose the minor weight turned round : it will imme- 

 diately begin to exert through the resistance to torsion, or elasticity, 

 of the single wire, a force which will tend to relieve or oppose, accord- 

 ing to the direction of the new application, the external force which 

 keeps the major weight detorted. Suppose the new force applied in 

 the same direction, so as to relieve the former. As the torsion of the 

 single thread increases — with the increase of the angle through which 

 the lower weight is turned — a point is at length reached when it 

 exactly relieves the whole of the external force applied to the upper. 

 Suppose this to occur when it has turned through an angle 0, i.e. 

 through an angle + 0, from the initial position. Then it is clear 

 that the force which turned the upper end of the single wire through 

 an angle 0, has been found equal to that which turns its lower end 

 through an angle 0, relatively, and + 0, absolutely. We have now to 

 consider what these forces are. 



Before doing so, it would be advisable to recognise the means pro- 

 vided for observing these angles. 



In some way, which there are no means of exactly discovering, the 

 designer or constructor has ascertained that by a certain apportion- 

 ment of lengths, weights, and thicknesses, the proportion of to 

 (which of course is a variable one) can be made 1 :3 when 0=90°. 

 The result of this is, that when + amounts to one complete revolu- 

 tion, alone is one quarter of a revolution. [In this position the re- 

 sistance of a bifilar suspension is a maximum. I do not know that it 

 has any strong advantage except what may turn on that. It is only 

 necessary to allude to the fact, to take occasion to add that it is cer- 

 tainly of no importance, either practically or theoretically, whether 

 is exactly or only approximately, equal to 90°. We shall see even- 

 tually that another consideration (perhaps not considered by the in- 

 ventor) entirely overrides the one mentioned. This by way of 

 parenthesis.] 



The head of the minor weight carries a small flat mirror, which faces 

 in the position of rest, as also after one revolution, a horizontal colli- 

 mator. The block of the major weight carries three similar mirrors, of 

 which one is parallel to the former (or may be made so) when in a 

 state of ease, and the others become so (under proper conditions) when 

 the major weight has been turned to the right or left through one 

 quarter of a revolution. These mirrors face the upper half of the 

 object-glass of the collimator. A fiducial mark in the focus of the 

 latter is seen by reflection from the mirrors when they are perpen- 

 dicular, or nearly so, to the line of sight. It will be necessary to 

 return to this in describing the intended observation. 



