MAGNETISM. 



63 



stated by the last of these observers that, 

 in the Isabella and Alexander, the bin- 

 nacle compasses of the two ships were 

 soon found to diiter very materially from 

 one another, in indicating the course 

 steered. The difference was frequently 

 one point, or eleven degrees and a quar- 

 ter. No dependence whatever could be 

 placed on the agreement of compasses 

 in different parts of the ships, or, of the 

 same compass with itself, if removed 

 but a few inches. Even in the neigh- 

 bourhood of the binnacles, the varia- 

 tion, as observed amidships, was from 

 8 to 10 greater than the result of 

 azimuths taken by a compass placed 

 between two and three feet on the lar- 

 board side, and an equal difference, in a 

 contrary direction, took place, on remov- 

 ing the compass to the starboard side ; 

 all of \vhich introduced great difficulties 

 in the ship's reckoning. 



(2-iS.) An extensive investigation of 

 the subject was now instituted by Mr. 

 Barlow, with a view of discovering some 

 principle of computation, or other 

 method, for correcting this source of 

 error, in all parts of the world. The re- 

 sults of the first experiments he made 

 for this purpose, were published by him 

 in 1820* ; and, in 1824, there appeared 

 a second and greatly extended edition of 

 the same work, developing the mathe- 

 matical principles which regulate the 

 action of unmagnetic iron upon a mag- 

 netized needle. His situation, as Pro- 

 fessor of the Royal Military Academy of 

 Woolwich, gave him the means of pur- 

 suing his experiments upon a very ex- 

 tended scale ; as he could procure, with 

 facility, considerable masses of iron, 

 such as balls and shells of every deno- 

 mination, and having that regularity of 

 figure which was most favourable to 

 the application of mathematical formulae. 

 As the inquiry is important, not merely 

 from its application to the subject of the 

 local attraction of vessels, but also in its 

 bearings on the whole theory of magnet- 

 ism, we shall briefly state the principal 

 results which he obtained. 



(249.) Mr. Barlow ascertained, that a 

 ball of iron produces no disturbance of 

 the compass needle, when the latter is 

 situated in any part of a plane passing 

 through the centre of the ball, and at 

 right angles to the direction of the dip- 

 ping needle, in the place where the ex- 

 periment is made. The angle of the 



* Under the title of ' An Essay on Magnetic At- 

 tractions,' 



inclination of this plane to the horizon 

 is, therefore, the complement to the 

 angle of the dip. In London, where the 

 latter may be taken at 70, this angle is 

 consequently 20. The section of this 

 plane of neutrality, as it may be called, 

 by a horizontal plane, passing through 

 the centre of the ball, will be a line 

 directed to the magnetic east and wesf. 

 If a hollow sphere, of considerable dia- 

 meter, be supposed to extend around 

 the ball, and to be concentric with it, 

 the plane above defined will, by its in- 

 tersection with the sphere, form a great 

 circle, which may be regarded as the 

 magnetic equator of that sphere, with 

 relation to the magnetic action of the 

 ball. 



(250.) Another plane of neutrality is 

 constituted by a vertical plane, also 

 passing through the common centre of 

 the ball and sphere, and including the 

 magnetic direction, that is, the line of 

 the^dip : this plane is evidently that of 

 the magnetic meridian ; and it also inter- 

 sects a great circle on the imaginary 

 sphere. 



We have termed these two planes the 

 planes of neutrality, in preference to 

 adopting the name of planes of no at- 

 traction, by which Mr. Barlow has de- 

 signated them, because, as Poisson has 

 remarked, it is not the whole of the 

 attractive force exerted by the iron ball 

 that vanishes in these planes ; but only 

 that part of this force which occasions 

 deviations in the natural position of the 

 needle, which, indeed, is the only force 

 of which we are now studying the effects. 

 Strictly speaking, however, there re- 

 mains another force, acting in a direc- 

 tion parallel to the dipping-needle, but 

 of an opposite nature to the action of 

 the earth, and tending, therefore, to re- 

 tard the oscillations of the needle. 

 There is, indeed, no plane in which the 

 attraction of a sphere, or, in general, of 

 any body magnetized by the earth's in- 

 fluence, becomes evanescent. 



(251.) In like manner, other meri- 

 dional great circles may be conceived 

 on the sphere cutting the equator at right 

 angles, and meeting at the two poles of 

 that equator; and the situation of any 

 point at the surface of the sphere, may 

 be designated by its distance from the 

 equator, measured on the meridional 

 circle which passes through the point, 

 and which may be defined its magnetic 

 latitude; together with its distance from 

 any one meridian, fixed upon as the first 

 meridian, measured on a smaller circle 



