86 Mr. R. W. Fox's Notice of an Instrument for ascertaining 



The instrument in question will, however, admit of the ap- 

 plication of an unequivocal test of any change in its magnetic 

 force. 



If the dipping-needle be made to assume a horizontal posi- 

 tion, by the proper adjustment of the deflector, or by weights 

 applied to its southern arms, it will retain very nearly this po- 

 sition, whatever way it may be turned; and it would quite do 

 so, were it not for the action of the terrestrial magnetism on 

 the deflector and needle, which tends to produce some modi- 

 fication of their forces, in a greater or less degree, according 

 to their relative directions : for example, if the north pole of 

 the needle be deflected upwards from its natural dip in this 

 latitude when in the plane of the magnetic meridian, by rais- 

 ing the north pole of the deflector under it (with the deflecting 

 tubes closed,) till the needle is perfectly horizontal and point- 

 ing towards the north, both the needle and deflector will have 

 their forces augmented by the action of the terrestrial mag- 

 netism ; and the latter more so than the former, because it 

 more nearly approaches the direction of the magnetic currents. 

 Turn the face of the instrument round 90°, when the ter- 

 restrial magnetism will have no effect in modifying the force of 

 the horizontal needle, though it will still add to that of the 

 deflector; in this case the needle will have a tendency to suffer 

 a slight depression. Let the instrument be turned 90° further 

 round till it is again in the plane of the magnetic meridian, but 

 with the north end of the needle pointing towards the south ; 

 the earth's magnetism will in this case tend to diminish the 

 force of the needle and of the deflector also, if it be at a 

 greater angle from the horizontal line than the complement of 

 the dip, and the depression of the north pole of the needle 

 will probably not be less, and may be more considerable than 

 when the instrument was at right angles to the plane of the 

 magnetic meridian *. This source of error may, however, be 

 compensated by reversing the direction of the deflector, and 

 drawing out the tubes: so that the mean results obtained in 

 both these ways, when the needle is at right angles to the plane 



* The modifying effect of the terrestrial magnetism is rendered more 

 apparent by attaching small weights to the north pole of the needle, and 

 raising the deflector till the needle becomes horizontal. 



Does it not seem to follow that the results obtained by the vibrations of 

 a masnetic needle are influenced, not only by the attraction of the ter- 

 restrial magnetism, but also by an adventitious force, which it imparts to the 

 needle in a greater or less degree in proportion as their relative directions 

 are more or less coincident with each other ? I do not know whether 

 this question has been duly considered, but it appears to me to be of some 

 importance that it should be taken into account in making comparative 

 observations in different latitudes with the horizontal vibrating needle. 



