36 MAGNETISM. 



habit of carrying a delicate pocket compass, which I applied 

 occasionally to the guns, staunchions, iron bolts, shot, &c., and I 

 invariably found a north and south pole in each separate article 

 of iron, whatever its form might be; I found that in our hemis- 

 phere the upper part of a gun, shot, or pig of ballast, attracted 

 the north point of the compass, and repelled the south point, 

 the lower part of the iron attracting the south point of the com- 

 pass-needle, and repelling the north point ; I also found, that 

 the reverse of this happened in places south of the magnetic equa- 

 tor, and, being aware of these circumstances, I was in the habit 

 of making allowances in steering a course. 



It is a generally received opinion, that the force of attraction 

 between the opposite poles of two magnets, is inversely as the 

 square of the distance between them : 



N S 



That is to say, if two magnets, situated with respect to bearing, 

 thus, that N, n, shall not be east and west from each other, but 

 that the distance N, s, is greater than N, S, then will the attrac- 

 tion n, S, be greater than that between N, s, because the force of 

 attraction, between opposite poles, is inversely as the square of 

 the distance between them, and consequently where the distance 

 is least, the force of attraction is greatest. 



If this principle be applied to the experiment made by Mr. 

 Barlow with the iron filings on the surface of the water, and ad- 

 mitting that the sphere, and filings were magnetical, bt/ position, 

 and having north and south poles, these poles, in the separate 

 particles of the iron filings, must have been so very near to each 

 other, that their attractive and repulsive forces, (with regard to 

 the iron sphere) would mutually destroy each other, and if we 

 consider the adhesive nature of the fluid which supported the 

 iron on its surface, and that the line, joining the poles of a par- 

 ticle of the iron, and the iron sphere, must have been a vertical 

 line, we need not be astonished that no motion was observed. 



To satisfy myself whether soft iron became magnetic by posi- 

 tion, with respect to the earth's axis, and by induction from 

 terrestial magnetism, I procured two bars of soft iron, each an 

 inch square and eighteen inches long, I knew that they would 

 act on a compass as magnets, agreeable to Mr. Savery's theory. 



