24- MR. p. BARLOW'S CORRECTING-PLATE 



sition of the iron and the compass in all his future inquiries ; 

 and to this happy and philosophical ideahe is doubtless indebted 

 for much of the remarkable success, with which his experiments 

 were ultimately attended. 



A compass placed in any point of the plane of no attraction, 

 or equator, of this sphere, will be uninfluenced by the iron 

 ball, and will preserve its natural magnetic direction. But as 

 soon as the compass is removed out of this plane, the needle is 

 found to deviate from its original bearing ; its south end being 

 drawn towards the ball when the needle is below the plane, and 

 its north end being drawn towards the ball when it is above the 

 plane. And in every case the deviation follows a determinate 

 law, so that the amount being given in any one case, it may 

 be computed for all other cases. 



This was an immense step towards the development of the 

 mathematical laws of magnetism, hitherto almost unknown, 

 but it was not all that was required. It was requisite also to 

 know what was the law observed at different distances. Sub- 

 sequent experiments enabled Mr. Barlow to demonstrate, that 

 while the position, as to latitude and longitude, was the same, 

 the tangent of the deviation was proportional to the cube of 

 the distance ; a law, however, which it is unnecessary to my 

 present object to pause in order to explain. Lastly; it re- 

 mained to be found, when the position and distance were the 

 same, what was the law of deviation, as it depended upon 

 the magnitude of the attracting body. In the pursuit of this 

 inquiry a most unexpected result was obtained. It appeared, 

 in the first place, that the tangent of deviation was propor- 

 tional to the cube of the diameter, which might naturally 

 be expected in solid balls, being merely a case of the law last 

 stated; but this law was found to obtain also in hollow shells: 

 in fact, the most solid ball was found to be no more than 

 equal in power to a shell of the same dimensions ; although 

 even the thickness of the shell should not exceed one-twen- 

 tieth of an inch. In other words, it was thus ascertained that 

 the magnetic power of masses of iron resides on the surface 

 only. And Mr. Barlow demonstrated, further, in a subsequent 

 series of experiments, that these laws have place not only in 

 balls and shells, but also in masses of iron of all forms and di- 

 mensions ; the centres of action, in all cases, being referred to 

 two points, (the two magnetic poles,) indefinitely near to each 

 other, in the general centre of attraction of the mass disturb- 

 ing the compass. 



These laws being determined, the application of means in- 

 dicated by them, to the correction of the local attraction of 

 vessels became equally rapid and certain. To repeat the Ian- 



