MAGNETISM. 



71 



right piece, which slides between two 

 grooves in the side of the box ; it con- 

 sists of an upright plane, P, having a 



Fig. 65. 

 S 



narrow vertical slit, S, in its upper 

 part ; below this is a circular aperture, 

 in which a convex lens is placed : a ho- 

 rizontal plane, H, proceeding from the 

 lower edge of the former, and also fur- 

 nished with a convex lens, and a mir- 

 ror, M, is placed behind P, and inclined 

 at an angle of forty-five degrees. By 

 means of this combination of lenses 

 with the mirror, the degrees on the card 

 are seen by reflexion, considerably 

 magnified, and in a reversed position, 

 together with the index on the ivory, 

 which is contiguous to the part viewed. 

 The image of the card, beinij produced 

 by one reflexion, is reversed ; on this 

 account it is requisite that the figures 

 expressing the number of degrees should 

 themselves be reversed, so that they 

 may read correctly when viewed through 

 the lenses. This sight may be raised 

 or lowered as much as is necessary to 

 adjust its focus ; and the whole is made 

 to turn back, by means of a hinge, so 

 as to lie in Ihe same plane with the box 

 when the instrument is placed in its 

 case, as represented in fig. 66, which 

 admits of its being carried in the pocket. 



Fig. 66. 



(273.) The following is the method 

 of using this instrument, when the azi- 

 muth of the sun is to be taken. Ele- 

 vate the object- sight, and turning it to- 

 wards the sun, slide the glass along it 

 till the line of light is thrown on the 

 ivory index. The sight next the eye is 



then to be adjusted, by raising it from 

 the position in which it lies when flat in 

 the case, till its horizontal lens is over 

 the edge of the card ; and the pro- 

 per focal distance obtained by sliding 

 it in the dove-tailed groove till the in- 

 dex line is seen distinctly. Next ob- 

 serve whether the line of light from 

 the sun seen on the piece of ivory 

 through the lens appears narrow and 

 well-defined ; and if it does not, incline 

 the sight furthest from the eye towards 

 the compass, till the requisite distinct- 

 ness is attained. Be careful that the 

 sight leans neither to the right nor left, 

 but is held perpendicular to the horizon, 

 in the direction between the sun and 

 the observer; for the neglect of this 

 precaution is the principal source of 

 error to be apprehended. Let the com- 

 pass be now inclined towards the ob- 

 server, so as to check the oscillations of 

 the card, by bringing it in contact with 

 the index and two pins fixed near it for 

 the purpose. Do this repeatedly, till 

 the card is steady, the compass being 

 sufficiently inclined from the observer 

 just to free the card from the index. 

 The line of light being then accurately 

 bisected by the index line, the degree 

 and fractional parts also indicated by 

 this line may be read off at the mo- 

 ment that an assistant takes the altitude 

 of the sun. If the card should happen 

 not to be perfectly steady, the mean of 

 its vibrations may be readily estimated. 

 The degrees on the card are read from 

 the north towards the east, and are car- 

 ried round to 360, in order to obviate 

 the possibility of error in this respect. To 

 the degrees and minutes thus obtained, 

 must be applied the correction written 

 on the card, and the result will be the 

 true magnetic position of the sun, from 

 which, and the observed altitude, the 

 variation of the needle may be obtained 

 in the usual manner. When the varia- 

 tion is to be determined for the purpose 

 of correcting the ship's course, it is 

 sufficient, and indeed necessary, that 

 the magnetic azimuths should be taken 

 without any reference to the local at- 

 tractions which may affect the needle ; 

 but for scientific deductions, after a cer- 

 tain number of observations have been 

 obtained with the ship's head in one di- 

 rection, she should be put on an oppo- 

 site course, and another set of observa- 

 tions taken frc;a the same spot: the 

 mean of the twu. results will be the true 

 variation of the , ;edle. 

 (274.) Captain Kater's azimuth com- 



