Bifilar or Horizontal Force Magnetometer. 



XXXI 



Table 4. — Observations of Deflection 



the Bifilar 



for the Determination of the Coefficient of 

 Magnetometer. 



No. 



of 



Series. 



Deflections of Bifilar Magnet. 



Deflections of Declination Magnet. 



Resulting 

 Value of k. 



Date. 



Distance. 



Deflection. 



Mean. 



Therm. 



Date. 



Distance. 



Deflection. 



Mean. 



Therm. 



1 I 



2 { 



3 I 



4 | 



1847. 

 May 11 



May 13 



May 12 



May 11 

 May 13 

 May 12 



May 15 

 May 15 



May 19 

 May 19 



Feet. 



2 " 2 {w 



2 7 ' E 

 2-7 | w 



3-267{^ 



" ff 



3-167^ 



4-583J^ 



6-917^ 

 8-209J^ 



« 92 {w 



Sc. Div. 



133-4 

 138-5 

 64-4 

 66-4 

 34-3 

 35-4 



118-4 

 119-6 

 60-9 

 60-7 

 21-0 

 21-0 



119-8 

 71-4 



125-1 

 42-9 



Sc. Div. 



1360 

 65-4 

 34-9 



119-0 

 60-8 

 21-0 



119-8 

 71-4 



126-0 

 43-0 



53-6 

 61-9 

 58-5 



53-9 

 60-9 

 56-4 



55-2 

 55-2 



1847. 

 May 14 



May 14 



May 14 



May 15 

 May 15 

 May 15 



May 15 

 May 15 



May 15 



May 15 



Feet. 



2-2 {* 



3-267J^ 



2 - 5 {w 



3-167{^ 



4 - 583 {w 

 6 - 9,7 {w 



8-209^ 



» {? 

 6-833J^ 



Sc. Div. 



94-30 

 92-25 

 45-85 

 45-00 

 24-20 

 23-70 



81-60 

 82-40 

 4205 

 42-75 

 14-34 

 14-58 



82-56 

 82-84 

 50-13 

 49-88 



102-48 

 103-28 



44-23 



62 43 

 30 33 

 16 6 



55 9 



28 31 

 9 43 



55 37 

 33 38 



69 11 



29 45 



62-6 

 62-6 

 62-6 



600 

 60-0 

 60-0 



61-5 

 61-5 



61-8 



61-8 



0-0001345 

 0-0001359 

 0-0001342 



00001350 

 0-0001364 

 0-0001346 



0-0001351 

 00001370 



0-0001350 

 00001357 



37. 5th, In May and August 1847, a theodolite magnetometer by Mr Jones of 

 London was converted into a unifilar horizontal force magnetometer, the suspended 

 bar having been deflected through an angle (u — 8) as in Dr Lamont's method, the 

 variations of horizontal force were deduced from the scale readings, reduced to angu- 

 lar measure by the formula 



aX 



IT 



= — cot (« — A) (AU — A d) + q (t — t Q ) 



where u is the angle which the deflected magnet makes with the astronomical meri- 

 dian (negative when to the east) and 8 is the westerly declination, a u being the arc 

 value of the change of reading, and a 8 the arc value of the simultaneous change of 

 declination obtained from the declinometer : q being the temperature coefficient of 

 the deflecting bar ( = 0*00021), £ the standard temperature of the deflecting bar, and 

 t the temperature of observation. 



In May 1847, only three comparisons of the two instruments were obtained 



