MAGNETISM. 



73 



any^other extraneous magnetic force. 

 This object can hardly ever be suffi- 

 ciently attained by balancing the needle 

 upon a point, as in the common com- 

 pass ; because, however small the fric- 

 tion may be, it is still a force which is 

 required to be overcome at the begin- 

 ning; of every new motion, and which 

 must even prevent all motion until the 

 moving power has increased to a cer- 

 tain amount. This objection does not 

 apply in the same degree to the suspen- 

 sion of the needle by a fine thread, which 

 is accordingly the best plan of construc- 

 tion for a variation compass. Care 

 should, of course, be taken that the force 

 of torsion be as small as possible. Mr. 

 Bennet proposed a spider's thread as the 

 best material for obtaining great delicacy 

 of suspension, and procuring the greatest 

 magnetic sensibility: for although twisted 

 through many thousand turns, it occa- 

 sioned no sensible deviation in a needle 

 suspended by it ; showing that its force 

 of torsion is insensible *. 



(284.) The thread should be con- 

 tained in a vertical tube, fitted to the 

 middle of the upper side of an oblong 

 box, the remaining parts of which are 

 to be completed by glass plates, for the 

 purpose of protecting the needle from 

 agitation by the air. With a graduated 

 arc adapted to each end of the needle, 

 and magnifiers to observe the exact po- 

 sition of the extremities when referred 

 to these arcs, this simple form of the 

 instrument, which is the one employed 

 by Captain Kater, is calculated to an- 

 swer every practical purpose that can 

 be desired. Its superiority to the ordi- 

 nary construction was shown on the oc- 

 casion of the late Northern Expeditions, 

 when it was found that the friction on 

 the metal point in the variation needle 

 belonging to Mr. Browne, made by Dol- 

 lond, nearly a foot in length, and sus- 

 pended in the usual manner by an 

 agate cap on a metal point, was, in 

 the high magnetic latitudes reached 

 by Captain Parry, too considerable to 

 be overcome by the directive power of 

 the magnet ; and accordingly it hap- 

 pened that at Winter Harbour the in- 

 strument was quite useless, while the 

 one furnished by Captain Kater still tra- 

 versed t. 



(285.) Considerable light, however, 

 may be thrown upon the causes that 



* Philosophical Transactions for 1?92, p. 81. 



t Parry's Journal of a Voyage for the Discovery 

 of a North- West Passage, &c. See Appendix, p. 

 cxvi. 



produce the minute diurnal or 'monthly 

 changes in the variation, by adopting 

 the expedient suggested by Mr. Bar- 

 low, and which we have already slightly 

 alluded to, $ 122. From a variety of 

 considerations, we are warranted in_'con- 

 cluding that the direction assumed by 

 the magnetic needle is the result of a 

 great number of magnetic forces acting 

 upon it, some of which are of a nature 

 more permanent than the rest. Thus, 

 while the average direction is the re- 

 sultant of some cause of very general 

 operation, affecting extensive portions 

 of the globe, many occasional changes 

 are effected by causes of a more tran- 

 sient nature, some of which are periodi- 

 cal in their influence, but of which 

 others are, as it were, accidental, or 

 at least very irregular and fluctuating 

 in their action. Innumerable obser- 

 vations have proved that the compass- 

 needle is more or less agitated during 

 the prevalence of the aurora borealis *. 

 Its deviation from this cause has been 

 known to amount to six or seven de- 

 grees. Volcanic eruptions have been at 

 various times observed to occasion con- 

 siderable disturbance in the position of 

 the needle : this was particularly noted 

 during the eruptions of Mount Hecla 

 and of Vesuviust . Atmospherical 

 changes, such as violent winds, or a fall 

 of snow, have, in like manner, been 

 known to affect the needle. The elec- 

 trical conditions of the atmosphere, and 

 especially those connected with the ap- 

 proach or occurrence of thunder-storms, 

 have a powerful influence on magnetic 

 polarity. 



(286.) It is evident that as the needle 

 in its ordinary states is urged to move by 

 a force resulting from the influence of 

 these variable forces, combined with 

 those that are of constant operation, the 

 effect of the former would be much 

 greater if the latter were withdrawn ; 

 and this can only be effected by neu- 

 tralizing the operation of these constant 

 forces. Mr. Barlow effected this by ap- 

 plying one or more magnets in the re- 

 quisite positions, so as to counteract 

 almost entirely the natural magnetic in- 

 fluence of the earth. In illustration of 

 which he gives the following example $. 

 Supposing that a finely suspended hori- 



* See Wargentin's Memoir in the Philosophical 

 Transactions for 1?51, p. 126. 



f The Abbe de la Torre observed changes of se- 

 veral degrees in the declination of the needle, during 

 an eruption of Vesuvius. 

 I t Philosophical Transactions for 1823, p. 327. 



