LAWS OF MAGNETIC ATTRACTION. 23 



after leaving Beachy Head in sight at 6 o'clock in the even- 

 ing, the vessel should have been wrecked upon the same spot 

 at 1 or 2 p clock m the morning, without the least apprehen- 

 sion of being at all near shore." 



Such being the alarming effects of the otherwise greatly 

 improved construction of ships, resulting from the extended 

 use of iron, Mr. Barlow undertook the investigation of the sub- 

 ject, with the view of correcting the errors thus introduced. 

 At this time, little or nothing was known of the mathematical 

 laws of magnetic attraction, on a knowledge of which, how- 

 ever, it was clear, that the discovery of a method of counteract- 

 ing the evil would immediately depend. 



It had previously been ascertained, indeed, that while a 

 compass needle was placed near the upper end of a bar of iron, 

 the north end was drawn towards the bar, and that near the 

 lower end, the south end of the needle was drawn towards it; 

 and it consequently followed, that there must be some interme- 

 diate point in which the effect of both ends was neutralized. It 

 was also known that a large mass of iron had a more powerful 

 attraction than a smaller mass ; and that the effect was greater, 

 as the distance between the iron and the compass was less ; but 

 no explicit and connected laws governing these effects had 

 been enunciated. To establish such laws was therefore the 

 first object of Mr. Barlow. He ascertained by experiments 

 with a solid iron ball, placing his compass about it in every 

 direction, that by causing the compass to descend in any ver- 

 tical line from above to below the ball, it always passed 

 through a point in which the iron had no effect upon it. By 

 continuing his researches, he found that all these points were 

 situated in the same plane ; which he called, therefore, the 

 plane of no attraction, and that this plane formed with the 

 horizon an angle equal to the complement of the dip of the 

 needle, descending from the north towards the south, whence 

 he called it the magnetic equator. That this ought to be the 

 case when the needle had its natural dipping position, might 

 easily have been foreseen ; because then the iron would be 

 symmetrically disposed with respect to the two poles of the 

 needle; that is to say, there would be an equal quantity of iron 

 operating on each pole ; but that it should still be the same 

 with the horizontal needle, was a fact as novel to Mr. Barlow 

 at the outset of his inquiries, as it proved to be important in 

 all his subsequent researches. Having traced this circle on his 

 iron ball, and assuming the direction of the dipping needle as a 

 principal axis to it, its extremities forming the north and south 

 poles, he was immediately in possession of an ideal magnetic 

 sphere, by which he might accurately indicate the relative po- 



