68 
View of Mr Barlow's Magnetical 
supply the place of the old water-casks. Moreover, the knees* 
sleepers, and, in some cases, even the riders are now of iron j 
hempen cables have been supplanted by those of iron ; and 
some attempt has been recently made to employ gun-carriages 
cf the same material. But of all innovations of this kind, the pa- 
tent capstan by Captain Phillips, a highly valuable construction, 
has, perhaps, from its form and situation, the greatest effect on 
the compass; indeed, its action is so powerful, as will be seen as 
we proceed, that, without the means afforded by Mr Barlow’s 
correcting plate, it must of necessity have been prohibited in all 
vessels of a smaller class than frigates. In the Griper, for ex- 
ample, the local attraction was 14° at east and west, making an 
extreme difference in the river Thames of 28°, which was redu- 
ced to about 1 6° by the removal of the capstan. 
Having thus made our readers acquainted with the nature of 
the errors which nautical men had presented for philosophical 
investigation, let us follow Mr Barlow in the experiments he un- 
dertook, with a view to discovering some means of correction. 
It should be observed, that, at the time of which we are speak- 
ing, little or nothing was known of the mathematical laws of 
magnetic attraction. It had been ascertained, that, while a com- 
pass-needle was placed near the upper end of a bar of iron, the 
north end was drawn towards the bar, and that, near the bottom, 
the south end of the needle was attracted ; and it consequently 
followed, that there must be some intermediate point, in which 
the effect of both ends was neutralized. It was also known, that 
a large mass of iron attracted more powerfully than a smaller 
mass ; and that the effect was greater, as the distance between 
the iron and compass was less r and amongst other ratios it had 
been stated, that the power varied inversely as the cube of the 
distance ; but still, no explicit and connected laws had been es- 
tablished : this, therefore, was the first object of the author. 
With this view, he procured a solid iron-ball, thirteen inches in 
diameter ; and placing his compass above this, he found, as in 
the case of the bar, that the north end of the needle was at- 
tracted by the ball ; that, when it was placed below it, the 
south end was attracted ; and that, by causing his needle to de- 
scend in any vertical, it always passed through a point where the 
iron had no effect upon it. The question then occurred, Are all 
