440 Sir G. C. Haughton o?i the Common Nature of 



against it. Either pole of the needle may be used, and the 

 bar-magnet may be made to influence it either by attraction 

 or repulsion, as is most convenient for the observation. The 

 operator will likewise find it an advantage to use a low seat, 

 as he will by that means be able to have his eye in the same 

 level as the object examined as often as it may be neces- 

 sary, particularly when minute objects are observed. 



AH that has just been said merely regards the mode of 

 avoiding a failure ; but if it is desired to measure the relative 

 magnetic intensities of the substances under examination, then 

 a graduated disc is to be employed, which may be placed 

 under the supporting needle, its N. and S. being in the line 

 of the magnetic meridian ; but a very convenient substitute 

 for these two will be found in detaching the bottom of one of 

 the compass-boxes, of which I shall immediately speak, covered 

 with its engraving of the rhombs and degrees, and cutting 

 away so much of it as might interfere with the approximation 

 of the different bodies. The nearer the needle can be brought 

 to 90^ by the attraction or repulsion of the bar-magnet, the 

 greater is to be considered the magnetic intensity of the object, 

 which is to be shifted after each measurement so as to increase 

 the angle of amplitude till it is found that the needle will no 

 longer attach itself, which consequently gives the limit of its 

 magnetic intensity. This being an exhausting and tentative 

 process, renders such measurements extremely tedious in many 

 cases, and yet it cannot be remedied ; for, owing to the time 

 lost in waiting for the oscillations of the needle to cease, as 

 well as for it to connect itself with the substance, and in the 

 various adjustments of the latter, much time is expended ; 

 added to which, the experiment must be often repeated in 

 difficult cases, to be quite sure that a failure has really arisen, 

 from a want of attractive affinity between the needle and sub- 

 stance. Nearly an hour has been consumed in an experiment 

 where much attention and delicacy of manipulation were re- 

 quired. 



Great care of course must be taken not to confound effects 

 purely electric with those that are magnetic. To prevent 

 such a possibility of error, it is only necessary to breathe upon 

 a suspected substance, which will effectually remove all free 

 electricity. Thus Iceland spar, which cannot be touched with 

 the fingers without becoming electric and remaining so for 

 days, may in the space of a ^evf seconds be rendered fit for 

 examination, as the humidity of the breath quickly evaporates 

 and carries off the superabundant electricity. 



It is well-worthy of remark, as showing how much time is 

 necessary, not merely for the exhibition of magnetic attraction 



