XXX Inteoduction to the Makerstoun Observations, 1845 and 1846. 



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 



= — cot (m — a) (a m — a 3) + q (( — ^o) 



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), to 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 

 during a moderate magnetic disturbance ; the results are given as a specimen of the 

 accuracy that may be expected from this method. 



1-. * Bifilar , 'Au-AS) /. , \ . 



Date. ,. i J " » .. nno ,cj t 9 (' — 'ft) * 



( orrectcd. x cot 39 40' ^ ^ "^ 



Sc. Di 



May 7<* 22^ O"" 4799 - 14 4288 25 16-82 48-4 



23 30 533-0 -14 29-56 25 9-86 +0007114 ^g.^ -0000063 00001328 

 8 1 32 571-9 -14 20 28 25 400 +0005331 ^g.^ -0000084 00001344 

 The mean of the two values of A; = 00001336. 



In August 1847, a series of coi^iparative observations were made of the bifilar 

 and a unifilar horizontal force magnetometer : these observations were made every 

 hour for three days ; the results were grouped so as to obtain the greatest differences 

 of readings for comparisons ; the mean angle of deflection of the unifilar (w— ^) was 

 equal to 65°. The final result of the whole groups was, that the changes of the uni- 

 filar scale readings were to those of the bifilar scale readings as 1 to 0-974, the value 

 of k for the unifilar being 0-0001389, therefore that of k for the bifilar = 0-0001353. 

 The changes of horizontal force from which this result was deduced were small. 



38. The following, then, are the values of A-, deduced by the five diff'erent pro- 

 cesses above : — 



Short deflector, E. and W. of bifilar magnet, and N. and S. of declination magnet,A; = 00001349 



N. and S E.andW A = 00001353 



Large deflector, E. N. and S A; = 0-0001360 



S. E.andW A; = 00001353 



Comparisons of unifilar and bifilar horizontal force magnetometers, A; = 00001353 



The mean of all the results gives A = 0-0001354 

 The adopted value of A; = 0000135 



