120 



ABSOLUTE OBSERVATIONS AND ADJUSTMENTS. 



The bifilar scale reading 400 was accordingly assumed to be = • 076200, and with the scale 

 value found from deflections as mentioned below, p. 124, table 3 was compu ted for the 

 reduction of the variation observations. 



It appears from Table 1 that the value of on regularly decreased throughout the year, an 

 assumption a priori probable, as the magnet was kept at a fairly even temperature, and never 

 received any shock or blow. 



In order to utilise observations of vibration unaccompanied by au observation of deflection, 

 and vice versa, so as to compare the observations with one another, and with the corresponding 

 bifilar readings, the value of m was assumed to diminish uniformly, and the amount t (3 m) of 

 the diminution after a time t, was obtained from the observed values of m, each value 

 yielding an equation of condition, of the form m = M — / (o m). 



The probable values of M and o m having been found from these equations, a value of m 

 was computed for every day on which an observation was made, and from it a value of X. 

 derived. These values being reduced to the standard bifilar reading, the mean of 23 vibration 

 observations was found to be • 076599, and of 19 observations of deflection -076621. Giving 

 half weight to the deflection observations, on account of their greater liability to error, the 

 mean amounts to • 076606. 



"When the corrections from Table 2 are applied to the bifilar readings, this mean becomes 

 •076578, thus agreeing very closely with the value found above. 



The probable error of a single observation of vibration is • 000052, and of an observation 

 of deflection • 00008. 



Table 3. 



Corrected Scale 

 Keadincr. 



Absolute 



Horizontal Force, 



C.G.S. 



Corrected Scale, 



Beading. 



Absolute 

 Horizontal Force, 



C.G.S. 



Div. 



— 6oD 



—500 



— 400 



— ,300 



— 200 

 — 100 



o 



o' 0-921 

 ■0607,3 

 •062,30 

 •06389 



■0^553 



•06720 

 ■06892 



Div. 



+ 100 

 + 200 

 + 300 

 + 400 

 + 500 

 + 600 

 + 700 



0^07067 

 •07247 



■°743i 

 •07620 

 ■07814 

 •08012 

 •08216 



Absolute Declination. 



Observations for absolute declination were made with the above-mentioned unifilar, the 

 declinometer being read simultaneously. 



Each observation consisted of three or more readings of the collimator magnet with its 

 "scale erect;" it was then turned 180° on its axis, and a like number of readings taken 

 with the " scale inverted." The torsion was always removed from the suspension thread 

 before commencing observations. 



The astronomical meridian was determined by star observations with the transit theodolite 

 to within a few seconds, and then indicated by fixed marks both north and south. As the same 

 pillar was used both for the transit instrument and the unifilar, the observed magnetic 

 declination could be referred directly to the meridian. 



The following table gives the results of these observations, the readings being reduced to 

 the declinometer scale reading 330. 



