all known Substances to the Magnetic Influence, ^c. 125 



would be little difficulty of giving them a directive power, which, 

 at considerable distances, in reference to their foe > 1 lengths, 

 would be very nearly, or almost exactly, the same; and, indeed, 

 were there some little difference in the magnetic intensity of the 

 two sets of bars, it would be no difficult matter to ascertain the 

 relative force of each, and to obtain sufficiently accurate results, 

 by calculation, accordingly. The appropriation of the results 

 obtained from the smaller set of magnets for the determination 

 of the distance of the larger magnets from the compass, when 

 the same deviation was produced, would be most simple — ^the 

 measure of the distance given by the smaller magnets being to 

 the real distance through the rock, as the focal length of the 

 smaller is to that of the larger magnets. If, indeed, the smaller 

 magnets were a foot, and the larger three feet in length, the 

 distances at which they would produce equal deviations, suppos- 

 ing their magnetic energy proportional, would be just as one to 

 three. 



Another method might be adopted for reducing the observed 

 deviation under a neutralizing or directing needle to the ratio in 

 the table of deviations, which is by finding the number of oscil- 

 lations of the compass needle in a given time, after its adjust- 

 ment in the mine, and by comparison with its ordinary vibra- 

 tions under terrestrial magnetism only, calculating the relative 

 magnetic forces contending against the action of the magnet *► 

 But this method being troublesome, the former is to be preferred 

 for ordinary practice* 



As in many cases, however, the comparative levels or direc« 

 tions of the two workings might not be satisfactorily known, a 

 single experiment, though it give the distance between the com- 

 pass and the magnet, will not be sufficient to determine the nearest 

 approximation of the two contiguous mines ; but the nearest 

 approximation, I apprehend, may be very well determined, and 

 at the same time, — a matter of no mean importance, — the com- 

 parison of level and direction, as well as the distance of the 

 workings. It may be useful, however, for the sake of perspi- 



• Here the intensities of the magnetic forces, under different circumstan- 

 ces, will be proportional to the squares of the numbers of horizontal oscilla- 

 tions in equal times. 



