258 C. F. GAUSS ON A NEW INSTRUMENT FOR OBSERVING 



ratus, and with the magnetometer of the observatory, have af- 

 forded the most satisfactory results. This application, however, 

 has lost much of its importance from the introduction of the 

 declination magnetometer, which gives the minutest changes 

 with all the accuracy that can be desired. 



This and other applications of the instniment, with the bar 

 in the reverse position, to which I shall hereafter return, must, 

 howevei', be considered as of minor importance ; the employ- 

 ment of the apparatus in the third or ti'ansverse position for ob- 

 servations of the intensity being far more important. If in pro- 

 ceeding from the direct position the magnetic bar is deflected 

 from the magnetic meridian by turning the stirrup, the whole 

 apparatus, in order to regain its equilibrium, must turn back 

 through a certain angle corresponding to the proportion of the 

 two directive forces ; the difference of the two angles will be the 

 deviation of the magnetic bar fi'om the magnetic meridian when 

 in the position of equilibrium ; and it may easily be arranged so 

 that this de\'iation shall amount to nearly 90 degrees, and thus 

 the advantages previously spoken of be gained. In this position 

 the apparatus is peculiarly well adapted for observing changes 

 of the intensity, which are immediately indicated by changes 

 of position. In regard to such changes as only take place in 

 long intervals, several circumstances must be attended to ; for 

 instance, it is requisite that from time to time we examine, by 

 known and appropriate means, whether and to what extent the 

 magnetism of the bar may have changed ; the variations of 

 temperatm-e must also be considered, both in their effect on the 

 magnetic state of the bar, and on the interval and length of the 

 suspending wires, and, consequently, on the directive force 

 arising from the mode of suspension. But with respect to the 

 irregular changes of the intensity in short intervals, this appa- 

 ratus performs the same service as the magnetometer does in 

 respect to similar changes of the declination ; and the mode of 

 observation with both instruments is the same. The changes of 

 intensity are obtained, expressed in parts of the scale, which, 

 however, may easily be reduced to fractions of the intensity itself. 

 Under the present relations of the apparatus, the 22,000th pai't 

 of the entire intensity answers to one di\asion of the scale. 



The experiments hitherto made with the apparatus, comprising 

 as yet but a very short period, have already indicated some im- 

 portant results. 



In the first place, the observations indicate the regular changes 



