27G W. WEBER ON THE ARRANGEMENT AND 



same time, as the figures must be covered either by the noniuses 

 of the one or of the other alidade, according as they are inside 

 or outside of the graduation. For this reason the tigurcs are 

 placed alternately on the inner and on the outer side, as exhi- 

 bited in PL XIV. fig. 2. 



Thc; fourth rotation is that of the two upper ends of the wires. 

 No mechanical arrangement is required for this rotation ; but the 

 bearer on the ceiling, by which the wires are carried and ad- 

 justed, is turned by the hand. As the bearer must be fixed to 

 the ceiling, no use is ordinarily made of this rotation ; but it is 

 so arranged in the first instance as to be in the most convenient 

 position for all pm-poscs. That position may be regarded as 

 most convenient in which the lower ends of the wire interfere 

 least with the mirror which is situated between them. It will 

 be evident that, in the various uses to which this instrument is 

 applied, if the bearer is not moved, the lower ends of the wire 

 are brought into various positions, while the mirror retains its 

 position between them nearly unchanged, being always directed 

 towards the scale. The two Avires, for instance, will sometimes 

 be in one vertical plane throughout their whole length ; some- 

 times they perform part of a revolution round each other, and a 

 vertical plane drawn through them will form with the fonner 

 one an angle, which is, however, always less than 90 degrees. If 

 it be now so arranged that in the first case the plane of the 

 wires coincides with the vertical plane of the optical axis of the 

 telescope, the one wire passes just as far from the mirror in front 

 as the other does behind, and both wires are as far as possible 

 from the mirror. If the instrument is then arranged for the 

 other use, the wires are brought nearer to the mirror, but not 

 so as to touch it, even if the mirror were larger than the inter- 

 vening space, because the rotation does not amount to 90°. It 

 is always less than 90°, because the directive force arising from 

 the suspension must be greater than the magnetic directive 

 force ; hence the moments of rotation ai'ising from the two forces 

 will only equilibrate when the wires undergo a smaller rotation 

 than the magnetic axis ; and since the latter, in the transverse, 

 must be 90° from its natural position, it follows that the rotation 

 of the AAires must be less than 90°. 



3. On the use of the Bifilar Magnetometer. 

 I shall in conclusion briefly notice the series of experiments 



