72 



MAGNETISM. 



pass is also well adapted for surveying, 

 for which object, indeed, it was origi- 

 nally invented. To apply it to this pur- 

 pose, nothing more is necessary than to 

 slide the frame containing the segment 

 of the glass cylinder to the top of the 

 sight, when the hair will be seen, which 

 must be made to bisect the object viewed 

 by direct vision at the moment that its 

 bearing is also read off by reflexion. 



(275.) There is also another mode of 

 using this compass, which may, per- 

 haps, be found more convenient and ac- 

 curate than that already described. It 

 is simply to turn back the reflecting 

 sight, and to view the line of light, and 

 read off the degrees by direct vision; 

 and it has this decided advantage, that 

 if the compass should not be in a hori- 

 zontal position, the observer may readily 

 perceive and correct the error. Some 

 care, however, is necessary not to mis- 

 take in reading the figures indicating 

 the degrees, they being inverted as 

 marked upon the card : this may be pre- 

 vented by viewing them also by re- 

 flexion. 



(276.) On approaching the north 

 pole of the earth, the north end of the 

 needle will incline downwards ; but the 

 card may again be readily balanced by 

 taking out the ring and glass, and at- 

 taching a small bit of wax to the south 

 pole of the needle. 



(277.) When it is considered how 

 great is the diminution of the power 

 with which the magnetism of the earth 

 acts upon the horizontal compass needle 

 in very high magnetic latitudes, the sa- 

 tisfactory results which have been ob- 

 tained, even under such extreme cir- 

 cumstances as those of the late arctic 

 voyages, in Davis's Straits, for in- 

 stance, and Baffin's Bay, from the em- 

 ployment of Captain Kater's azimuth 

 compass, which gave correct observa- 

 tions when other instruments became 

 useless, afford the best testimony of its 

 excellence, and of the precision which 

 may be expected from its employment 

 in the ordinary course of observation *. 



(278.) In some azimuth compasses, 

 for which patents have been taken, a 

 triangular glass prism is substituted for 

 the mirror in the above instrument, act- 

 ing evidently on the same principle of 

 reflexion, and evidently borrowed, with 

 a trifling alteration of form, from the 

 original invention of Captain Kater. 

 Coloured glasses are sometimes pro- 



* See Captain Sabine's observations in the Philo- 

 sophical Transactions lor 1819, p. HI. 



vided for making observations on the 

 sun ; they are placed so as to be readily 

 interposed between the eye and the 

 nearest sight when wanted, and are 

 removeable at pleasure when not re- 

 quired. 



$ 4. Of the Variation Compass. 



(279.) A magnetic needle intended 

 to indicate the minute changes that 

 take place in the direction of terrestrial 

 magnetism, should, as already noticed 

 ( 242), be of somewhat greater length 

 than an ordinary compass, in order that 

 the extent of the variations of angular 

 position may be more conspicuous. 



(280.) For the same purpose, the fol- 

 lowing method was practised by Du 

 Hamel : At each extremity of a long 

 needle, a slender, pointed piece of steel 

 was erected perpendicularly, which 

 served as sights for observing its posi- 

 tion with reference to the divisions of a 

 graduated limb, six feet in length, fixed 

 to a pillar at the distance of nearly sixty 

 feet from the needle, and in the direc- 

 tion of its axis *. 



(281.) Analogous to this was the con- 

 struction employed by Mr. Prony, con- 

 sisting of a long magnetic bar, on which 

 was fixed a telescope, moving along 

 with it ; its motion being observed by 

 looking through it at a distant object, 

 the image of which would have a cor- 

 responding motion in the field of the 

 telescope. Humboldt, who made many 

 observations with this instrument, 

 considered it to be a very accurate me- 

 thod. 



(282.) But a point of much greater 

 importance is that the magnetism of the 

 needle should be uniform, and that its 

 magnetic axis should remain perma- 

 nent. Hence its form should be of the 

 simplest kind, such as that of a slender 

 needle of nearly equal diameter through- 

 out, and magnetised with great care. 

 The observation of minute changes of 

 position may be made with sufficient 

 accuracy by means of a magnifying 

 glass; which expedient will supersede 

 the necessity of employing sights, or of 

 giving to the needle any extraordinary 

 length. 



(283.) Another material point is to 

 obtain great delicacy of suspension, so 

 that the needle shall immediately obey 

 the slightest change of direction in the 

 force of terrestrial magnetism, or of 



* Histoire de 1'Academie Royale des Sciences de 

 Paris, for 1772, part ii., p. 50. 

 f Biot, Traite de Physique, torn, iii., pp. 143, 144. 



