MAGNETICAL INSTRUCTIONS. 11 



The Inclination as determined at sea is observed with Fox's circle in accordance with the 

 instructions, Appendix 3 ; care being taken to repeatedly determine the index correction due to the 

 separate needles, with face of instrument both east and west, by comparison with the results made on 

 land by Barrow's circle, i.e., with the results determined by needles whose poles were reversed and 

 the Inclination observed in the eight usual positions of the circle and needle. Note. As the index 

 correction is liable to vary as a function of the Inclination it should be determined in different 

 inclinations. 



The Declination at land stations can be determined with great accuracy by aid of the apparatus 

 and special collimator magnet, attached to the Unifilar magnetometer. (See instructions, Appendix, 

 Section I. Xo. 1.) It is an observation requiring, however, some time and much delicacy of mani- 

 pulation. 



As an auxiliary in the determination of this magnetic element, and to take its place when time 

 or circumstances press, a Rater's azimuth compass of five inches diameter is furnished, with which the 

 sun's magnetic bearing can be obtained by a process that obviates prism error. 



This compass admits of being accurately levelled, a most necessary precaution, as the principal 

 source of error to be apprehended in its use is from the sight vane leaning to the right or left, and not 

 perpendicular to the horizon in the direction between the sun and the observer. Attached to the 

 hinged sight vane is a short brass sliding frame, which carries the segment of a glass cylinder. By 

 means of this piece of glass, when presented to the sun, the rays are collected into a linear focus ; the 

 line of light, which must appear narrow and well defined, being thrown (by sliding the glass along and 

 inclining the sight vane) on a piece of ivory fixed so as just to come over the outer edge of the gradu- 

 ated circle of the card. This piece of ivory is marked with a line to serve as an index for reading off, 

 by means of the prism, the graduated circle of the card. 



This compass can also be used in the ordinary way for taking bearings by simply sliding the 

 frame containing the segment of the glass cylinder to the top of the sight vane, when the hair will 

 be seen which is used to bisect the object viewed by direct vision. 



For the due preservation of the cap and pivot of this delicate instrument the sight vane (which 

 moves on a hinge) can be folded down on the glass cover, and when thus folded it raises the needle 

 of the compass by means of a lever under its centre, so as to press it against the glass cover, and 

 prevent its moving. To prevent therefore the sudden jar of the jewelled cap on the pivot when about 

 to use the compass, the hinged sight vane should be gently raised till the card duly finds its rest. 



The Declination (or Variation) as determined at sea will of course be observed in the usual 

 manner at the ship's Standard compass ; but it is necessary that the index correction of this compass 

 and the several cards employed should be occasionally determined by a comparison of the values of 

 the Declination observed by them on land with that determined by the Unifilar magnetometer, or 

 Kater's azimuth compass. This necessity for obtaining the index correction arises from the circum- 

 stances that though the Standard compass and its cards leave the compass observatory at Deptford in 

 perfect and reliable adjustment, yet from usage sometimes rough incidental to long sea service, the 

 prism may be slightly misplaced, the cards slightly shrunk or distorted, or the sight vane slightly out 

 of adjustment : all causes affecting exact results. 



