MAGNETISM. 



01 



The pupil of the eye being bisected 

 by the upper edge of the prism, as in 

 the Camera Lucidu, the object and that 

 part of the circumference of the card 

 on which the degrees are marked, are 

 seen at the same time ; the former by 

 direct vision, the latter by reflexion from 

 the internal surface of the inclined face 

 of the prism: and thus the coincidence 

 of the two may be accurately noted. 

 A prism of the same kind is also ap- 

 plied in Gilbert's patent Azimuth Com- 

 pass. 



(-242.) The Variation Compass, de- 

 signed to exhibit the diurnal changes of 

 variation in the horizontal magnetic 

 needle, has generally a needle of much 

 greater length, than those of other kinds 

 of compasses ; and as it is not required 

 to move round the whole circumference, 

 the box, instead of being circular, is 

 oblong, so as to admit of a deviation of 

 only 20 or 25 degrees from the middle 

 line. A vernier scale, with a magnifier, 

 is usually applied in order to estimate 

 the changes of position of the needle 

 with greater precision. 



$ 2. Of the Local Attraction of Vessels. 



(243.) The indications of the mari- 

 ner's compass at sea are liable to error 

 from a cause which, till lately, had 

 never been supposed capable of affecting 

 the needle. It consists in the attraction 

 which the large quantity of iron, con- 

 tained in various parts of the ship, exerts 

 upon the magnetic needle : for although 

 the action of each individual piece of iron 

 may, at the distance at which it is placed, 

 be quite insensible ; yet the united action 

 of the whole quantity dispersed in every 

 part of the vessel, may amount to a 

 considerable sum, and occasion a very 

 perceptible deviation of the compass 

 from its true position in the magnetic 

 meridian. This will happen more espe- 

 cially in ships of war, which contain a 

 large number of guns, of iron-shot, and 

 of water-tanks, and various parts of the 

 frame-work of the ship which are now 

 made of iron. 



(244.) If we suppose each particle 

 of iron to exert a certain attractive 

 force upon the magnetic poles of the 

 compass needle, according to a certain 

 law, hereafter to be determined, it 

 is easy to understand how the com- 

 bined effect of all these forces may be 

 considered as equivalent to one simple 

 resultant force acting in a certain direc- 

 tion. If the quantity of iron be con- 

 sidered as equally distributed on both 



sides of the ship, and the compass be 

 placed, as is usual, in the binnacle, in 

 the after part of the ship, this resultant 

 force, which represents the combined 

 action of the iron, will be situated in a 

 vertical plane passing through the com- 

 pass, and through the axis of the ship, 

 and will, moreover, have a certain in- 

 clination to the horizon. In the northern 

 regions of the globe, the inductive in- 

 fluence of the earth on unmagnetic iron 

 consists in carrying the southern pola- 

 rity upwards, and the northern polarity 

 downwards, ($ 107,) in a ^direction pa- 

 rallel to that of the dipping'needle. The 

 action on the compass of a piece of 

 iron thus brought into a state of in- 

 duction will, therefore, be precisely 

 similar to that of a magnet having the 

 position of the dipping needle, and 

 placed at a considerable distance from 

 the compass. If it be placed, with re- 

 lation to the compass, exactly in the 

 magnetic meridian, (that is, to the mag- 

 netic north or south of the compass,) it 

 can have no effect in disturbing its po- 

 sition. This will generally be the case 

 when the course of the ship coincides 

 with the magnetic meridian, and the 

 needle of the compass is in the direction 

 of the axis of the ship. But if the ship's 

 head be turned to the eastward, and the 

 resultant force of the iron in the ship be 

 directed in a line downwards from the 

 compass, that force will be represented 

 by a magnet placed in the same oblique 

 line ; and the south pole of that magnet, 

 being uppermost, will act with most 

 power, and will attract the north pole of 

 the compass needle, causing it to deviate 

 towards the east. The same magnet, 

 placed to the westward of the compass, 

 which would correspond with the ship's 

 head being turned to the west, would oc- 

 casion a westerly deviation of the com- 

 pass needle. In the southern hemi- 

 sphere, when the inductive influence of 

 the earth has a contrary direction, the 

 opposite effects would result from the 

 action of the iron in the ship ; for the 

 action would then be represented by a 

 magnet, having a position with respect 

 to its poles, the reverse of what it had in 

 the former case. 



(245.) The earliest record of any ob- 

 servation of the effect of this local at- 

 traction of vessels, occurs in the voyages 

 of Captain Cook ; but the reason of the 

 deviation of the needle does not appear 

 to have been suspected. The first dis- 

 tinct statement of the real cause of this 

 anomaly is contained in a report from 



