ON A NEW MAGNETICAL INSTRUMENT. 229 



In the preceding observations a telescope of law power was 

 employed, and the arc-value of a single division of the scale 

 (which was at the distance of eighteen inches from the mirror) 

 was 3'-98. The differences of the observed and calculated results, 

 therefore, do not in general exceed the amount which may be 

 fairly ascribed to errors of observation ; and the accordance is 

 sufficient to establish the fact, that the changes of the induced 

 force of the bar are, within the observed limits, proportional to 

 those of the inducing action. It is important to note also that 

 the changes of the induced force, produced artificially in these 

 experiments, are much greater than any which are likely to arise 

 from the variations of the vertical component of the earth's 

 magnetic force, and therefore that the experiments may be 

 regarded as severe tests of the performance of the instrument. 



The preceding observations further showed, that the changes 

 in the inducing force were imtantly followed by their effeets upon 

 the suspended magnet; so that the changes of induced force 

 required no appreciable time for their development. It remained 

 only to ascertain, in a somewhat fuller manner, how far the bar 

 was susceptible of minute magnetic changes, from very small 

 variations of the acting force. For this purpose, a series of 

 readings of the scale was taken, the inclination of the bar to the 

 vertical being altered by half a degree between the consecutive 

 readings. The mean difference of the successive readings was 

 found to agree, very exactly, with the calculated difference ; 

 while the partial differences deviated from the mean by an amount 

 not exceeding the limits of error of observation. It may be 

 presumed, therefore, that the changes of the induced force in the 

 iron bar are continuous ; and, accordingly, that the sensibility of 

 the instrument is only limited by the optical power, which is 

 applied to observe the changes of position of the suspended 

 magnet.* In the experiments above described, the arc- value of the 

 divisions of the scale was nearly 4' : with the modifications since 

 introduced into the reading part of the apparatus, the scale 



* Against this conclusion is the fact, that considerable changes in the induced force 

 of the bar seem to be attended with some permanent changes of polarity ; and it may be 

 presumed that the same thing will take place, in a proportionate degree, with tho 

 minute changes induced by tho variations of the earth's force. It remains for future 

 examination to determine how far such permanent changes, if they occur, may impair 

 the accuracy of the results. 



