WEBER ON A TRANSPORTABLE MAGNETOMETER. 585 



placed inside, and the box closed again. When observations are made, 

 this box serves for suspending a second needle, the time of vibration of 

 which is required for the measurement of the absolute intensity; this 

 second needle/, g is provided at both ends with mirrors, one of which 

 serves for observing the scale. The needle rests on two supports h, k, 

 attached to a small measuring bar 7«, n, over which passes a thread 

 carrying the weights p, q, which serve to increase the moment of inertia 

 of the oscillating needle. The needle can be turned in the supports A, h, 

 and niay be reversed; rendering it available, in absolute measurements 

 of the declination, as an auxiliary needle, when the instrument repre- 

 sented in fig. 2. is used, the needle of which is not reversible. For 

 this purpose, instead of a needle with a mirror, one with a collimator, 

 fig. 13, may be jjlaced in h, k. It consists of a magnetic steel tube 

 a, b, c, d, carrying at the end a, c, an achromatic object-glass ; and at its 

 other extremity a sliding tube of brass e, f, g, h, provided with a glass 

 micrometer in the focus of the object-glass. It will be seen also by fig. 3. 

 that this needle is suspended to two threads, the upper points of attach- 

 ment of which are r and s. The threads are conducted over a roller z to 

 give them equal tension, and are united in one from u to v, forming an 

 unifilar suspension, which may be converted into a bifilar by opening 

 out the apparatus a,, fi, y, S, which is done by pressing down the knob w 

 by the screw t, and disengaging the threads from the pins x, y, as repre- 

 sented in fig. 9. Fig. 3. is also half size. 

 Fig. 4. A, is the theodolite carrying two telescopes and two scales; one tele- 

 scope and one scale serve for observing the unifilar magnetometer B, 

 and the other telescope and scale for observing the bifilar magnetometer 

 C. The figure gives the angles which the instruments ought to form 

 with each other. 

 Fig. 5. is a graphical representation of the variations of the declination ob- 

 served on the 13th of April, 1839, at Gottingen, simultaneously in the 

 magnetic observatory, and with the transportable magnetometer. 

 Fig. 6. is the tripod on which the magnetometer, fig. 2, is placed and le- 

 velled. 

 Fig. 7. is the apparatus required for the experiments of deflection, a, b, c, d 

 is a copper disc which fits on to the tripod, fig. 6 ; e,f, g, h, and k, I, m, n, 

 are arms screwed to the copper disc at e,f and h,l; one arm carries the 

 telescope p, q, to which the scale r, s is attached, and upon which the de- 

 flecting bar a, v is to be laid ; the other arm carries a tube on which the 

 deflecting bar is also laid, but which could not be conveniently repre- 

 sented in the figure. Between e,f anA k, I the magnetometer (fig. 2.) 

 is placed. 

 Fig. 8. is a smaller side-view of the magnetometer represented in fig. 2, in 

 its proper relative position to the measuring apparatus, fig. 7, and rest- 

 ing on the tripod, fig. 6. In this view the needle is seen only by its 

 circular cross-section, and the glass plate is shown, in the side of the case 

 which permits the image of the scale, reflected from the mirror, to be ob 

 served with the telescope. 

 Fig. 9. is explained in the description of fig. 3. 



