USE OF THE BIFIIiAR MAGNETOMETER. 275 



ployed at the same time, so as to free the indices without turning 

 the mirror from the scale. 



The third rotation is that of the stirrup with its alidade, on 

 the circle upon which it rests. The directive force of the wires 

 acts immediately on the circle to which the suspension screws 

 are fixed : the directive force of magnetism acts immediately on 

 the stirrup in which the magnet-bar is placed. When, there- 

 fore, these two directive forces form an angle with each other, 

 the two parts upon which they act will have a tendency to 

 move in opposite directions. That no such displacement of 

 the parts may occur, they are made to slide on each other with 

 so much friction, that the two directive forces, when forming a 

 large angle with each other, may not be able to overcome it. For 

 a similar reason it was provided in the unifilar that the aUdade 

 of the stin-up should be placed on the outermost margin of the 

 circle, so that the friction produced by its pressure might act 

 with the greatest leverage. The same has been done with the 

 bifilar, where this provision is much more essential and im- 

 portant, the forces which tend to displace the two parts being 

 much more powerful. Further, we must be able to measure 

 with great exactness this rotation, on which depends the angle 

 which the two directive forces form with each other. The sim- 

 plicity of construction of the bifilar consists chiefly in this cir- 

 cumstance, that the same circle and graduation serve for mea- 

 suring both the second and the third rotation. For this reason 

 the alidade of the stirrup is also furnished with two noniuses. 

 The instrument consists, therefore, of a circle with two ahdades, 

 which may be used independently of each other. In order that 

 this independent use may never cause the two alidades to inter- 

 fere, the one is situated beneath, and the other above the circle. 

 But since each alidade is pro\'ided with two noniuses, and all 

 four are to move on the divided hmb of the circle, which is its 

 upper surface, the inferior alidade embraces the margin of the 

 circle and forms noniuses which abut at the outer margin, whilst 

 those of the upper aUdade, in order not to come in conflict with 

 those of the lower, abut on the inner margin. The noniuses of 

 the upper aUdade can thus pass by those of the lower one, and 

 even an interval may exist between them, which, however, must 

 be smaller than the length of the divisions on the circle. Thus 

 the graduation of the circle serves two purposes, the one not in- 

 terfering with the other, only it cannot serve both purposes at the 



