112 
LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
results which will be obtained with the smaller apparatus described by Lieut. Riddell 
in his “ Magnetical Instructions for the use of Portable Instruments,” &c. In this 
apparatus the suspended and deflecting magnets are respectively 3*0 and 3*67 inches 
in length. Meanwhile we may derive, as a provisional value, the arithmetical mean 
of the four mean results already stated ; allowing to each an equal weight, we have, 
fMean 4*54 
Ross, in 1840-1841, 15-inch magnets, 4*573' 
Kay, in 1841, do. do. 4*553 
Kay, in 1842, do. do. 4*513 
Kay, in 1843, 12-inch magnets, 4*520 
which, with the other necessary data stated in the preceding Number of these Contri- 
butions, would give the value of the total intensity at Hobarton 1*81 to 1*372 in 
London*. 
* Since these pages were written I have received the details of the observations of ten distinct determina- 
tions of the absolute horizontal intensity at the magnetic observatory at Hobarton, made in August 1 843 with 
deflecting and suspended magnets respectively of 948 inches and 7*50 inches in length. The deflecting 
distances were the same throughout, being 3'2893 and 4‘3393 feet. The calculation of these observations not 
having been yet received from Lieut. Kay, the results have been computed by Lieut. Riddell, R.A., F.R.S., 
so far as the materials hitherto furnished permit. They give the value of X' ; — being the absolute horizontal 
intensity (X), uncorrected for the difference in the magnetic moment of the deflecting bar produced by the earth’s 
inducing action in the different positions in which the bar is placed in the experiments of deflection and in 
those of vibration; viz. 1° perpendicular to the magnetic meridian, and 2° in the plane of the meridian. We 
owe the suggestion of a correction due to this cause to Dr. Lamont : but the necessary data for computing it, 
for the particular bars employed by Lieut. Kay on this, or on the former occasions, have not yet been received. 
Observations made at the Cape of Good Hope and at Woolwich, with similar bars, have given results which 
show that the correction may possibly prove to be of nearly the same amount for the larger and smaller bars, 
in which case the relative values will be but little affected, and we may estimate that the value of X at Hobarton 
will be about 0‘02 less than X'. In the expression which has been employed in these Contributions for the 
absolute horizontal intensity (1‘82 + e at Hobarton and 3*72 + e at London, e being a small quantity to be 
supplied hereafter), the correction here referred to will form a portion of e. The following Table exhibits the 
abstract of the observations made in August 1843 with 948 and 7‘50 inch bars. 
Deflecting Magnet. 
Values 
of X'. 
Bifilar Magnetometer. 
Gottingen Mean Time. 
No. 
Value 
of m'. 
Temperature 
during 
deflection. 
£=000229. 
q= 000224. 
Reading. 
Temp. 
d h 
1843. Aug. 20 19*0 
948 inch. 
6-256 
54-6 
4-5052 
1654 
52-0 
21 11*5 
948 inch. 
*259 
49-6 
•5034 
168-6 
49-1 
21 16-5 
94 8 inch. 
•251 
51-9 
•5043 
165-3 
49-1 
21 19*5 
94 8 inch 
•261 
53-7 
•4993 
168-3 
50-0 
22 11-0 
948 inch. 
•227 
48-0 
•5177 
165-4 
49-3 
22 19*5 
948 inch. 
•243 
54-5 
•5025 
164-6 
50-7 
23 10-8 
948 inch. 
•259 
50-7 
•4884 
161-0 
51-2 
23 18 1 
94 8 inch. 
•244 
52-4 
•5005 
162-2 
51-0 
23 19*1 
948 inch. 
•240 
52-0 
•4982 
163-9 
51-3 
25 11-4 
948 inch. 
•252 
49-4 
•4953 
165-3 
51-5 
6-249 
51-7 
4-5015 
165-0 
50-5 
The mean value of the results, 4 - 501, is considerably different from the mean deduced in the text from all 
