ON SELF-RECORDING MAGNETOGRAPHS. 223 



afford us corrections, which, when applied to our monthly absolute determi- 

 nations, should bring them all to the same value : in other words, the self- 

 recording magnetograph affords us the means of eliminating the changes that 

 are constantly taking place in the value of the magnetic declination. Should, 

 however, the torsion of the suspension thread of the declination magnetograph 

 have become changed to any extent, our corrected monthly determinations 

 will no longer have the same value. 



We are thus presented with a test, by means of which we may ascertain 

 whether change of torsion in the suspension thread, or some other circum- 

 stance, such as the change in position of some neighbouring mass of iron, 

 has affected our magnetograph. The following results show that the mag- 

 netograph herein described has stood this test in a very satisfactory manner: — 



Time of observation of Declination reduced by 



absolute declination. magnetograph to Jan. 1858. 



1858 January 21 56 27 



February 21 54- 47 



March 21 56 2 



April 21 56 59 



May 21 56 33 



August 2155 38 



September 21 57 1 



October 21 55 4 



1859 October 21 57 7 



November 21 56 38 



December 21 54 53 



Before concluding this part of the subject, I may remark that the magneto- 

 graphs are merely intended to serve as differential instruments ; so that, in 

 addition to their employment, absolute values of the magnetic elements require 

 to be taken from time to time. On this account also, although it is very de- 

 sirable to have, if possible, no torsion in the thread of the declination mag- 

 netograph, and no iron in its neighbourhood, yet the value of the result does 

 not depend so much on the entire absence of these sources of error as in the 

 constancy of the effects which they produce. The greatest caution should 

 therefore be exercised in excluding any hygrometric influence which might 

 change the torsion, and the greatest pains taken to prevent any shifting of 

 iron in the neighbourhood of the instruments. 



2. Horizontal-force Magnetograph. 



We have in this case two things to determine, viz. the temperature cor- 

 rection, and the value of one inch on the cylinder in parts of force. With 

 regard to the first of these, the most trustworthy method is to make the ob- 

 servations themselves determine their own temperature correction by means 

 of comparing together two periods, for which the average temperature is 

 different, while the average horizontal force is known to be the same for both. 

 It is, however, advisable that the temperature correction of the horizontal- 

 force magnet should be well determined in the ordinary manner before 

 mounting it. With regard to the scale coefficient, or value of 1 inch in parts 

 of force, it may be well to exhibit in detail the process by which the scale 

 coefficient of the present horizontal-force magnetograph has been determined. 



There are two methods by which the scale coefficient is determined. In 

 the first of these, let v denote the angle which the plane of the upper extre- 

 mities of the wire makes with that of the lower; Iv the change, in parts of 



