12 SCIENTIFIC APPARATUS. 



force is very regular in its action, and may be accurately deter- 

 mined by proper experiments. 



This arrangement, which is called the Bifilar suspension, was 

 invented by Gauss and Weber for their magnetic apparatus. It 

 was afterwards used by Baily in his experiments on the attraction 

 of balls. 



14. METHODS OF READING. 



The observed position of the indicating part of an instrument is 

 recorded as the " Reading." To ascertain the position of the 

 indicating part of the instrument various methods have been 

 adopted. The commonest method is to make the indicating part 

 in the form of a light needle, the point of which moves near a 

 graduated circle. The position of the needle is estimated by 

 observing the position of its point with respect to the divisions of 

 the scale. By giving the needle two points at opposite extremities 

 of a diameter, and observing the position of both points, we may 

 eliminate the errors arising from the want of coincidence between 

 the centre of the graduated circle and the axis, of motion of the 

 needle. 



This is the method adopted in ordinary magnetic compasses. 

 As it is necessary for freedom of motion that the point of the 

 needle should not be in actual contact with the graduated limb, 

 the reading will be affected by any change in the position of the eye 

 of the observer. The error thus introduced is called the error of 

 Parallax. In some instruments, therefore, the observation is made 

 through an eye-hole in a definite position. A better plan, how- 

 ever, is to place a plane mirror under the needle, and in taking 

 the reading to place the eye so that the needle appears to cover 

 its own reflexion in the mirror. 



15. SPIEGEL- ABLESUNG, OR MIRROR-READING 



A still more accurate method is that invented by PoggendorfT, 

 and used by Gauss and Weber in their magnetic observations. A 

 small plane mirror is attached to the indicating piece, so as to 



