584 WEBER ON A TRANSPORTABLE MAGNETOMETER. 



creased until t' is about = 0*6871 t. The torsion circle must 

 then be turned until the middle of the scale appears in the field 

 of view of the telescope, and the time of vibration t" observed. 

 The magnetometer is then in the transversal position proper for 

 obser\dng the variations of intensity, and the value of the scale 

 divisions may easily be calculated from the observed times of 

 vibration t, t', t"; namely, if o- denote the arc-value of a division 

 of the scale in parts of radius, the value of a division of the 

 scale, in pai'ts of the whole horizontal intensity, is 



;..=yO-4).i.. 



Explanation of the Figures, Plate XXV. 



Fig. 1. a,b, c, dls a vertical longitudinal section of the copper case of the mag- 

 netometer, with the needle e,f suspended by a silk thread g, h. The 

 needle is seen to be pierced through its length, and provided at the ex- 

 tremity/with a lens ; it is inclosed in a copper tube k, I, m, n, and can 

 be turned by means of a key o, p, which is accessible by an opening in 

 the copper case. In doing this the copper tube is held by two screws 

 q, r, and two pi-ojections s, t. The mirror 2i, v is seen above the copper 

 case, near the axis of rotation of the needle. A dotted line indicates how 

 the telescope of the theodolite, fig. 10, is directed, both to the needle and 

 to the lens at its end/, and also to the mirror ?/, v. It is also seen how 

 the inclination of the mirror may be regulated by the screw tv, that 

 the image of the scale placed above the telescope at a, fig. 10, may 

 appear in the field of view. This fig. is half the size of the instrument 

 itself 



Fig. 2. represents a magnetometer, which differs from the one just described 

 in not being adapted for complete measurements of the declination. 

 The collimator is omitted, and the needle cannot be reversed. The 

 mirror a, b, c, d is inclosed in the copper case, and is parallel to the 

 plane of the magnetic mei'idian ; the inclination of the mirror is regu- 

 lated by the screw at e ; the copper case forms an unbroken damper 

 round the needle, except at the aperture for the suspension thread ; the 

 mirror is observed through a glass plate in one of the sides of the copper 

 case. This figure is also half size. 



Fig. 3. a, b, c, d, e represents the wooden case, in which either of the instru- 

 ments shown in figs. 1. and 2. are packed for travelling. The lid, with 

 the tube a, b, c which is fastened to it, is taken off, the instrument 



