16 
LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
I have allowed, therefore, a loss of magnetism in this needle equivalent to 5 S, 4 in 556 
seconds at all the stations subsequent to Mazatlan, and have accordingly deducted 
*00848 from the logarithms of the squares of the times of vibration of No. 7- at those 
stations. 
We now proceed to a similar general intercomparison of the needles at Martins 
Island. 
Table IV. — Intercomparison of the Intensity Needles at Martins Island. 
Quotients. 
5. 
7. 
8. 
9. 
11. 
12. 
13. 
Mean. 
Panama and Martins Island 
0-975 
0-983 
0-981 
0-981 
0-981 
0-983 
0-978 
0-980 
Differences from the Mean 
- -005 
+ -003 
r— l 
o 
o 
+ 
+ -001 
+ -001 
+ -003 
— -002 
Here, with the exception of No. 5, which appears to have sustained a slight loss 
of magnetism, the agreement of the quotients shows the general steadiness of the 
magnetic condition of the needles. In the case of No. 5. the difference does not ex- 
ceed the limits of occasional error of observation ; the results with this needle sub- 
sequently to San Bias may not deserve to be regarded as fully equal in value to those 
of the other needles ; but the amount of error in the final determinations hazarded 
by retaining its results independent of correction is insignificant. 
As this Table includes the whole interval between the observations at Panama in 
1839, and those at Martins Island in 1840, we may regard it as substantiating the 
general steady magnetic condition of the needles in the whole of that interval, with 
the exception of the changes already noticed in Nos. 7- and 8, which have been traced 
to the period of their occurrence, and their amount examined and allowed for. 
The times of vibration at all the stations visited subsequently to March 1839 were 
taken in arcs commencing with 20° : the time of the chronometer was noted at every 
10th vibration during 300, and the mean time of 200 vibrations derived from ten 
partial results, i. e. from the 0th and 200th, the 10th and 210th, the 20th and 220th. 
No satisfactory experiments having been made to determine the individual coeffi- 
cients in the correction for temperature of these needles, I have taken an arbitrary 
coefficient for that purpose, being the arithmetical mean of the coefficients experi- 
mentally ascertained for the twenty-nine needles specified in the following list : — 
000165 Hansteen 
*00019 
Lenz 
*00025 
Lenz 
•00026 
Lenz 
•00029 
Lenz 
•00016 
Lloyd 
•000254 
Lloyd 
•000248 
Lloyd 
. . . . Needle 
. . . . Needle 
. . . . Needle 
. . . . Needle 
. . . . Needle 
. . . . Needle 
. . . . Needle 
Phil. Trans., 1828. Art. I. 
n 
2 
Mem. de 1’ Acad. Imp. de 
St. Petersbourg, 1824. 
. 4 ^ 
L (4) Trans. R. I. A., vol. xvii. 
^ >Brit. Assoc. Report, 1835. 
L(6)J 
