570 WEBER ON A TRANSPORTABLE MAGNETOMETER. 



§ II. Description of the several parts. 



Fig. 1 represents the vertical section of the magnetometer in 

 the direction of the magnetic meridian. 



The magnetic bar which forms the needle is bored through- 

 out its length, and the opening which is turned towai'ds the 

 telescope is provided with a lens, in the focus of which at the 

 other end there is a cross of wires. This cross of wires is seen 

 in the telescope, when (as is required for determining the true 

 azimuth in the measurement of the absolute declination) it is 

 adjusted to distant objects, and then directed to the lens. This 

 arrangement was proposed by Airy, to make it possible to dis- 

 pense with the mirror, and to be able to make, with the same 

 telescope, and without displacing the eye-glass, the astronomical, 

 geodesical, and magnetical observations required in measuring 

 the absolute declination. In making this measm-ement the 

 needle must be reversed ; but in the reversal the optical axis 

 must not alter its relative position in respect to the needle ; 

 this is effected in the closed case by means of a key, turned 

 on the outside, and causing the needle inside to perform half 

 a revolution round its axis of length. But this arrangement 

 is inapplicable to observations which require great changes 

 in the position of the needle, as in the experiments of vibration 

 and deflection in the measurement of the absolute intensity. It 

 therefore appeared advantageous to employ also a mirror, placed 

 in the same manner as in the bifilar magnetometer, close to the 

 axis of rotation of the needle, and above the copper case, and 

 available hovrever great the deflections may be. 



The copper case is seen to have three openings : the first is 

 into a space containing the mirror, and closed towards the 

 theodolite by a plate of glass, through which the light can pass, 

 in the direction shown in the plate from the scale, to the mirror, 

 and thence back to the telescope of the theodolite. The other 

 two openings are nearly at the same height as the magnetic 

 needle and the telescope of the theodolite. The light entering 

 through one of these apertures illuminates the cross of Avires 

 which is stretched across the hindmost end of the hollow needle, 

 passes on to the lens at the other end, and thence, parallel to 

 the horizontal direction marked in the figure, to the telescope of 

 the theodolite with which the cross of wires is observed. The 

 needle, bored tluoughout its length, is made accurately cylin- 



