GO 



MAGNETISM. 



tion of its directive force, are productive 

 of an evil of a much more serious kind, 

 than any that can arise from mere un- 

 steadiness : for it is evident that the 

 very same cause which makes the com- 

 pass partake of the irregular motions of 

 the ship, forces it, in the same degree, 

 to deviate from its proper position in 

 the magnetic meridian. While the card 

 remains apparently steady, the steers- 

 man will pursue his course, unsuspicious 

 of danger, until the first warning of his 

 error may, perhaps, he the sudden ap- 

 pearance of a shore, from which he had 

 imagined himself at a considerable dis- 

 tance. The real remedy for the incon- 

 venient vacillations of the compass is 

 that we have already pointed out, 

 namely, the accurate adjustment of the 

 point of suspension in the line of the axis 

 of rotation of the gimbals, which, as we 

 have before observed, ought to intersect 

 one another at right angles, in that 

 same point. In addition to this, it may 

 be advantageous to increase the weight 

 of the magnet, provided its directive 

 force be at the same time augmented. 

 This may be effected by employing, as 

 the compass needle, a magnet of greater 

 thickness, or by combining several 

 needles together, laying them parallel to 

 one another; for if both the magnetic 

 power and the weight, (and conse- 

 quently the friction,) increase in the 

 same proportion, the directive power 

 will remain the same as before ; and the 

 compass, thus constructed, being hea- 

 vier, will be deranged to a less extent by 

 the same disturbing force : and when 

 deranged, will be brought back by the 

 directive force to its proper bearing, with 

 the same facility as in an instrument of 

 the ordinary construction. 



(239.) It is to be recollected that if 

 a needle, in its unrnagnetic state, be so 

 constructed as that it shall be accurately 

 balanced when resting on a point at its 

 centre, and shall maintain itself in a 

 horizontal position, and if it be after- 

 wards magnetized, the influence of ter- 

 restial magnetism will cause it to as- 

 sume an inclined position, one of its 

 ends preponderating, as if it had ac- 

 quired additional weight. In order, 

 therefore, to restore the equilibrium, 

 and bring it back to the horizontal 

 plane, it will be necessary to add a cor- 

 responding weight to the other end of 

 the needle. 



The degree of inclination in the un- 

 balanced needle, depends upon the 

 amount of the dip, which, as we have 



seen, varies in different parts of the 

 world, according to the situation of the 

 place with regard to the magnetic poles 

 of the earth. Hence, when the compass 

 is transported to a distant part of the 

 globe, a different adjustment must be 

 made of the weight applied to correct 

 the tendency to dip. These adjustments 

 are best effected by means of a sliding 

 piece of brass placed under the needle, 

 and the position of which may change, 

 according to circumstances, on the one 

 side or the other, to any distance that 

 may be necessary In long voyages, 

 during which the changes of latitude are 

 considerable, the position of this regu- 

 lating weight requires to be frequently 

 shifted, in order to accommodate the 

 needle to the varying changes of incli- 

 nation incident to the variations of lati- 

 tude. 



(240.) The Azimuth Compass differs 

 from the ordinary Mariner's Compass 

 only in the circumference of its inner 

 box being provided with sights, through 

 which any object, "either in the horizon, 

 or above it, may be seen, and its bear- 

 ings from the magnetic points of the 

 compass determined, by reference to the 

 position of the card, with respect to the 

 sights. For this purpose the whole box 

 is hung in detached gimbals, which turn 

 on a strong vertical pin, fixed below the 

 box, which is thus capable of being 

 moved round horizontally, and of the 

 sights being directed to whatever object 

 is to be viewed through them. On one 

 side of the box there is usually inserted 

 a nut or stop ; which, when pushed in, 

 presses against the card and stops it ; 

 this is done to enable the observer to 

 read off the number of degrees of the 

 card, which correspond with an index, 

 or perpendicular line, drawn in the in- 

 side of the box. They are also some- 

 times read off by means of a wire stretch- 

 ing from one sight to the other across 

 the centre of the card. 



(241.) Analogous to this instrument is 

 the land or surveying-compass, which is 

 also furnished with sights, and means for 

 reading off the degrees on the card. This 

 latter object is effected in a very inge- 

 nious manner, by a contrivance of Mr. 

 Schmalcalder, for which he procured a 

 patent. The card is balanced in the usual 

 manner, and contained in a round brass 

 box, with two sights, the one to which 

 the eye is applied being furnished with 

 a triangular prismatic lens, and the 

 other being an open sight, with a vertical 

 horse-hair lineextending along its middle. 



