74 



MAGNETISM. 



zontal needle, under the natural in- 

 fluence of the earth, makes one vibra- 

 tion in two seconds ; and that by mask- 

 ing the terrestrial influence by magnets 

 properly adjusted, the time of vibration 

 is increased to eight seconds ; then it 

 would follow that the directive power 

 was reduced to one-sixteenth of the 

 former ; and, consequently, that any 

 lateral magnetic force acting upon the 

 needle would produce an effect sixteen 

 times greater than before; so that if 

 the former were twelve minutes, the 

 new effect on deviation might be ex- 

 pected to amount to between three and 

 four degrees, and therefore be such as to 

 admit of distinct and satisfactory obser- 

 vation. Thus he found that when the 

 needle was kept in its natural position, 

 and then deprived of nearly the whole 

 of its directive power by bringing a 

 magnet near it, the daily variation might 

 be magnified almost to any amount. 

 The same result was obtained when the 

 north pole of the needle was directed to 

 the south, east, or west, or, indeed, any 

 required position, at least within certain 

 limits. With this view, Mr. Barlow first 

 deflected the needle, by the repulsion of 

 a magnet, into a certain position, and 

 then, by means of another magnet, mo- 

 dified its directive power in the same 

 way as when it was in its natural posi- 

 tion in the magnetic meridian. 



(287.) Thus, by combining different 

 sets of observations, in different posi- 

 tions of the needle, information may be 

 obtained as to the direction as well as in- 

 tensity of the extraneous forces that in- 

 terfere with the general directive in- 

 fluence of the earth. Mr. Christie, in 

 prosecuting these investigations, pre- 

 ferred applying two magnets, placed 

 one above and the other below, and on 

 different sides of the needle, in the line 

 of the dip, or that in which it would 

 arrange itself if freely suspended by its 

 centre of gravity, instead of retaining 

 them in the same horizontal plane with 

 the needle, conceiving that a more 

 equable distribution of the forces acting 

 on the needle would thus be obtained ; 

 for a portion of the forces acting upon 

 the horizontal needle in the line of these 

 magnets would be destroyed, and it 

 would still be acted upon by forces in 

 the same direction as before, but of less 

 intensity; whereas by even applying 

 the poles of two magnets to the corre- 

 sponding poles of the needle, and in the 

 same plane with them, the horizontal 

 directive force of the needle would be 



diminished by increasing the angle which 

 the resultant of the terrestrial forces 

 and those of the magnet made with the 

 horizon, and which would be nearly 

 equivalent to increasing the angle of the 

 dip. This arrangement also procured 

 the further advantage of obtaining va- 

 rious modifications of effect, by altering 

 the distances of the neutralizing mag- 

 nets from the needle, whereby inferences 

 might be deduced as to the variations 

 in the intensities of the deflecting forces 

 occasioning the deviations of the needle 

 at different times. It would exceed the 

 limits of this treatise to attempt even a 

 short abstract of the mode of investiga- 

 tion pursued by Mr. Christie in this in- 

 quiry, and for the details of which we 

 must refer our readers to his paper in 

 the Philosophical Transactions*. The 

 general results to which he arrived have 

 already been given in $ 123, 124. 



(288.) In observations for determin- 

 ing the exact variation, great care should 

 be taken that the compass employed be 

 unaffected by any local causes of at- 

 traction from iron in the neighbour- 

 hood. In the account given of the me- 

 teorological instruments used at the 

 Royal Society's house t, Mr. Cavendish 

 points out the method he employed in 

 order to ascertain whether this cause of 

 error existed ; and if so, to determine 

 its amount. He removed -the variation 

 compass from the apartments of the 

 Society, into a large garden belonging 

 to a house in Marlborough- street, 

 about a mile and a quarter to the west 

 of Somerset House, where there seemed 

 to be no danger of its being affected by 

 any iron-work. Here it was placed ex- 

 actly in the meridian, and compared 

 for a few days with a very exact com- 

 pass, placed in an adjoining room, and 

 kept fixed constantly in the same situa- 

 tion. It was then removed back to the 

 Society's house, and compared again 

 with the same compass. By a mean of 

 these observations, the difference be- 

 tween the position in the two stations 

 was ascertained, indicating the amount 

 of the local influence of the iron in the 

 house and adjacent buildings, and con- 

 sequently the error of the instrument. 



5. Of the Dipping- Needle. 



(289.) The principle on which the 

 dipping-needle acts has already been 



* For 1823, p. 342. 



f Philosophical Transactions for 1776, p, 391. 



