10 THE VOYAGE OF H.M.S. CHALLENGER. 



furnished to be used on land ; and if the Inclination is at the same time observed, the Total Force in 

 absolute measure is determined.* 



Observations to determine the relative values, or the ratio of the Total Force at different stations 

 are made with the needles of Mr Fox's dip circle on a statical principle devised by that gentleman. 

 This instrument is consequently available for prosecuting a magnetic survey on laud as well as for its 

 more general adaptation at sea, provided that the Intensities observed with it can be referred to a base 

 station where the absolute determination of the Intensity has been effected. 



The Inclination or Dip as determined at land stations is observed with Barrow's circle (furnished 

 with two reversible needles) in accordance with the instructions, Appendix 2 ; attention being paid 

 to paragraph 9 thereof. 



Especial care should be taken to preserve the axles of the needles of this instrument from rust ; a 

 serious defect, which is somewhat difficult to obviate in tropical climates without due caution.-f- It is 

 desirable, therefore, that occasionally a few series of observations should be made in different azimuths, 

 for the purpose of testing the axles of the needles. 



In the ordinary mode of determining the Inclination, the final value corresponds to the mean of 

 eight observed results. The observations are made with the face of the circle and face of the needle 

 in different directions, in order to eliminate the error arising from the deviation of the line joining 

 the zero points of the circle from the horizontal position, and the error arising from the deviation of 

 the magnetic axis from the axis of form ; the object of repeating the observations with the poles of 

 the needles reversed, is to eliminate the error arising from the non-coincidence of the centre of gravity 

 with the centre of motion. 



For the purpose then of testing the axles of the needles (and the limb of the instrument) if 

 i) denote the observed Inclination of the needle, 6 the Inclination sought, a the azimuth of the 

 vertical circle 



tan 6 = tan rj cos a. 



The true Inclination may be inferred also from the observed Inclination of the needle, in any 

 two planes at right angles to one another, without the knowledge of the angle a according to the 

 formula 



COt 2 9 = COt 2 t) + COt 2 1)'. 



The difference between the mean of the results obtained by observation in different azimuths, and 

 the result obtained by observations in the magnetic meridian, would be a constant correction for the 

 errors of axle (and limb). 



The relations between the several forces are as follows : 



Horizontal Force x nat. secant of Inclination = Total force. 



x tangent = Vertical force. 



Total Force x cosine = Horizontal force. 



x sine = Vertical force. 



t Paraffine oil is stated to be a good preservative against rust ; greasy matter must, however, first be removed, or 

 the oil will not remain on. In a few days after coating a large portion evaporates or runs off, leaving a protecting film. 

 When the needles are required for use the film can be removed by dry warm blotting paper ; the axles of the needles 

 can be cleaned in soft cork previously perforating the cork with a small pin ; the grit of hard cork is liable to groove 

 the axle. Mercurial ointment should be carefully avoided as a preservative. 



