1i22 Rev. Mr Scoresby on the Universal PermeahiUfy 



It still remains for me to describe the requisite apparatus, 

 with the preparation of the magnets, and to offer a few sugges- 

 tions for carrying into the practice of mining, &c. the principles 

 developed by the preceding investigations. 



The apparatus necessary for the practical objects in view are 

 chiefly a pair of bar magnets, tempered throughout, of B feet or 

 upwards in length ; another pair, say of 12 inches, correspond- 

 ing in quahty, temper, and proportions; a compass (the one 

 employed for mining operations will do), with two or three other 

 articles, of simple structure, hereafter to be described, which 

 may be made by any ordinary artizans. The preparation and 

 determination of the powers of the magnets will be the first step 

 in the business. 



The bars having been magnetized to saturation, will require 

 to have their directive powers and foci of attraction determined. 

 Previous to this, it will be prudent to place them upon one ano- 

 ther for a few moments, with similar poles contiguous, not in 

 contact, but separated by a sheet of paper, or other thin sub- 

 stance, in order to disperse or neutralize any excess of magnetic 

 energy which they might not be able permanently to retain. 

 Without this or some similar precaution, the intensity of the 

 magnetic energy of the bars will be liable to diminution in the 

 course of the experiments, so as to prevent satisfactory or in- 

 deed useful results. The position of the foci of attraction may 

 then be easily determined. And in accomplishing this, for ordi- 

 nary practice, it will only be necessary to try the deviation at the 

 first three or four lengths. Assuming the focal position, as in the 

 experiments heretofore made, at ^^^th of the whole length from 

 each extremity, we shall generally, in properly magnetized and 

 well-proportioned bars, be very near the truth. 



But, by the way of example, let the position of the foci of 

 attraction of a three-feet bar, or of a pair of such magnets in 

 combination, be required. The assumed position of the foci of 

 attraction, at ^^xh the length within each extremity, will give 

 9. feet 6 inches for the focal length, and 3 inches for the distance 

 of each focus from the extremity. The first focal length, be- 

 cause of the measurements being from the centre of the com- 

 pass to nearest end of the magnet, instead of the nearest focus, 

 will be 9. feet 6 inches — 3 inches = 2 feet 3 inches. The se- 



