MAGNETISM. 



67 



shifted for every different position of 

 the ship, which would, of course, be 

 impracticable. He therefore had re- 

 course to the following expedient, which 

 was found to answer perfectly under all 

 circumstances of situation. Since it is 

 possible to place a ball of iron in the 

 same line of direction, with regard to the 

 compass, as that in which the com- 

 bined action of the iron of the ship is 

 exerted, and to bring it to the exact dis- 

 tance at which its action shall be equal 

 to that of the ship's iron, it is obvious 

 that a ball so placed will, instead of de- 

 stroying the deviation of the compass, 

 double its amount; and that this will 

 be the case under all circumstances, 

 and in every part of the world. Instead, 

 therefore, of fixing the ball, let its pro- 

 per place be first determined, and the ball 

 itself laid aside; then, at any time when 

 it is desirable to ascertain what effect 

 is due to the magnetic attraction of the 

 ship, let it be applied in the situation so 

 determined, and observe how many de- 

 grees it draws the needle of the compass 

 from the direction it had previously 

 to the application of the ball. This will 

 be the amount of the actual deviation 

 produced by the iron of the ship ; and 

 the correction in the course of the vessel 

 may be applied accordingly. Strictly 

 speaking, it is not the angle of deviation 

 which is doubled by the action of the 

 ball, but the tangent of that angle ; but 

 as, in small angles, the tangents are 

 very nearly in the ratio of their arcs, 

 they may in most cases be taken, with- 

 out sensible error, as the same. 



(263.) As the effect to be obtained 

 depends on the surface, and not on the 

 mass of the iron which acts, Mr. Bar- 

 low has found it more expedient to em- 

 ploy plates of iron, instead of balls. The 

 form he recommends is a double plate, 

 composed of two thin plates of iron, 

 screwed together in such a manner 

 as to combine any strong irregular 

 power of one plate, with a correspond- 

 ing weak part of another; by which 

 means a more uniform action is ob- 

 tained. These plates are of a circular 

 form, twelve or thirteen inches in diame- 

 ter, with a hole in their centre, through 

 which is passed a brass socket, with an 

 exterior screw; a brass nut, about an 

 inch and a half in diameter, screws on 

 the exterior of each end of the socket, 

 thereby pressing the plates together; 

 with an interposed thin circular piece 

 of board, which is intended to increase 

 in some degree the thickness of the 



plate, without adding to its weight. It: 

 would appear also that the compound 

 plate is more powerful when the two, of 

 which it is formed, are thus separated 

 from each other *. The proper position 

 of the plate, with regard to the compass, 

 must be ascertained by trials on shore ; 

 comparing its effects, in different rela- 

 tive situations, with the observed devia- 

 tion of the compass on board the ship. 



(264.) Although the method proposed 

 by Mr. Barlow be exceedingly inge- 

 nious, and will, no doubt, to a certain 

 extent, prove highly useful, several 

 causes exist in practice which must 

 interfere with the regularity of its ope- 

 ration. Changes of temperature will 

 probably affect the compass-needle, the 

 compensating plates, and the large 

 masses of iron contained in the ship, in 

 very different degrees; and many of 

 the latter bodies will be more or less 

 susceptible of acquiring permanent mag- 

 netism in the different circumstances in 

 which they are placed. In the course 

 of a long voyage, extending to very dif- 

 ferent latitudes, these causes are liable 

 to considerable variation, and must in- 

 troduce a degree of uncertainty in the 

 amount of the changes induced. Still, 

 however, the method of Mr. Barlow 

 will furnish a most valuable approxi- 

 mation to the correct determination of 

 the influence which the ship exerts on 

 the needle of the compass. Certain it is 

 that the proper estimate of the disturb- 

 ing force arising from this cause has, of 

 late years, acquired increased import- 

 ance from the very large proportion of 

 iron now employed in the construction 

 of ships of war, and of the machinery 

 for their guidance. Independently of the 

 guns, shot, and iron water-tanks, the 

 knees of the ship, the capstans, and 

 cables are now made of iron, so that the 

 whole forms a very large and powerful 

 magnetic mass. 



(265.) In all situations, but more 

 especially in high magnetic latitudes, 

 experience has shown the advantage of 

 adopting an expedient originally sug- 

 gested by Captain Flinders: namely, 

 the selection of some particular spot in 

 the ship as the permanent position of a 

 standard compass, in which it should 

 be invariably placed for use, whether in 

 observing azimuths, or bearings of land, 

 or in directing the ship's course-: so 

 that if, on any particular occasion, it 



* Barlow's Essay on Magnetic Attractions. Se- 

 cond edition, p, 100. 



F2 



