xxx INTRODUCTION TO THE MAKERSTOUN OBSERVATIONS, 1845 AND 1846. 
bar having been deflected through an angle (wu — 5) 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 
x 
=— cot (wu — A) (Au — Ad) + q(t — %) 
where wis the angle which the deflected magnet makes with the astronomical meri- 
dian (negative when to the east) and 6 is the westerly declination, su being the are 
value of the change of reading, and 4 6 the are value of the simultaneous change of 
declination obtained from the declinometer : g being the temperature coefficient of 
the deflecting bar (=0-00021), t) 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. 
eee i 3 hats ae D t q (t—to) k 
Se. Div. ° ’ ° 7 
May 7422" 0™ 4799 —14 42°88 25 16°82 48-4 ‘ 
23 30 5330 —14 29-56 25 986 +9°007114 42, —0-:000063 0-0001828 
Bi 1.82. 5yg 02 1denes 25k noes dg-1,. — 9 000084 
The mean of the two values of =0-0001336. 
Date. 
In August 1847, a series of comparative 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—0) was 
equal to 65°. The final result of the whole groups was, that the changes of the uni- 
filar seale 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 & for the bifilar = 0-0001358. 
The changes of horizontal force from which this result was deduced were small. 
38. The following, then, are the values of k, deduced by the five different pro- 
cesses above :— 
Short deflector, E. and W. of bifilar magnet, and N. and 8. ofdeclination magnet,4=0-0001349 
moe hacseen sweat INE ERAGINS) © On syaeyajnentoncoccanece” LOSE RAED fis neckonaaGecc ce MOReRRRadere adc UFO NOH SIE 
Large detiectors:E5 jy | Tater nsccrhestscrc acts DNtaM CMEC Rciaccaceec cask oorcae deat k=0-0001360 
smiresPreveoberes S. Pee eae ee EAD Wee itrdrecctteccsss+ccss-.e2ee =O OOO os 
Comparisons of unifilar and bifilar horizontal force magnetometers, k=0-0001353 
The mean of all the results gives 4£=0-0001354 
The adopted value of k=0:000135 
