224 Instructions for the Scientific Eitpedition 



ACCOUNT OF THE MAGNETICAL INSTRUMENTS EM- 

 PLOYED, AND OF THE MODE OF OBSERVATION 

 TO BE ADOPTED, IN THE MAGNETICAL OBSER- 

 VATORIES ABOUT TO BE ESTABLISHED BY HER 

 MAJESTY'S GOVERNMENT. 



The elements on which the determination of the earth's magnetic 

 force is usually based are, the declination, the inclination, and the 

 intensity. If a vertical plane be conceived to pass through the di- 

 rection of the force, that direction will he determined when its in- 

 clination to the horizon is given, as well as the angle which the 

 plane itself forms with the meridian ; and if, in addition to the.«e 

 quantities, w»-likewise know the number which expresses the ratio 

 of the intensity of the force to some established unit, it is manifest 

 that the force is completely determined. 



For many purposes, however, and especially in the delicate re- 

 searches connected with the variations of the magnetic force, a dif- 

 ferent system of elements is preferable. The intensity being resolved 

 into two portions in the plane of the magnetic meridian, one of them 

 horizontal and the other vertical, it is manifest that these two com- 

 ponents may be substituted for the total intensity and the inclina- 

 tion ; while, at the same time, their changes may be determined 

 with far greater precision. The former variables are connected with 

 the latter by the relations 



X — R cos ff, Y = R sin ; 

 in which R denotes the intensity, X and Y its horizontal and ver- 

 tical components, and Q the inclination ; and the variations of d and 

 R are expressed in terms of the variations of X and Y by the for- 

 mulae : 



rf = I sin 2 



(dY_dX\ 



— -cos^0_ + sm-0-^. 



As the instruments destined for the observation of these elements 

 (with a set of which each observatory is furnished) are, for the most 

 part, novel in form, it will be useful to give a somewhat detailed ac- 

 count of their construction and various adjustments, before enterin<»' 

 on the i^lan of observation to be pursued. 



Declination Magnetometer. 



Construction. — The essential part of the declination magnetometer 

 is a magnet bar, suspended by fibres of untwisted silk, and inclosed 

 in a box, to protect it from the agitation of the air. I'he bar is a 

 rectangular parallelopiped, 15 inches in length, |ths of an inch in 

 breadth, and ^th of an inch in thickness. In addition to the stirrup 

 by which the bar is suspended, it is furnished with two sliding pieces^ 



