MADE BY CAPTAIN BACK DURING HIS LATE ARCTIC EXPEDITION. 411 
times of vibration of the needle No. II. in the plane of the meridian ; and in some cases 
they are quite incongruous. If we suppose that the needles retained their intensities 
unimpaired from the time of the last observations at Fort Reliance until the obser- 
vations were made in London, that is, if we take the results of the latter observations 
as standards for comparison, the agreement will in general be greater ; but still 
similar incongruities will exist. The results thus obtained are given in the following 
Table : 
Place of observation. 
Needle No. 1. 
Needle No. 3. 
Lozenge-needle. 
Measure of 
intensity. 
Ratio of in- 
tensity to that 
at London. 
Measure of 
intensity. 
Ratio of in- 
tensity to that 
at London. 
Measure of 
intensity. 
Ratio of in- 
tensity to that 
at London. 
London 
•131253 
•160675 
1-00000 
1-22416 
•137573 
•161929 
•178122 
•146653 
•086601 
•085249 
•185352 
1-00000 
1-17704 
1-29475 
1-06600 
0- 62949 
0-61966 
1- 34730 
•799485 
•959637 
•505126 
1-22769 
1-00000 
1-20032 
0- 631815 
1- 53560 
Fort Reliance 
Musk-Ox Rapid 
Rock Rapid 
Point Beaufort 
Montreal Island 
Point Ogle 
Whatever may be the law according to which the terrestrial magnetic intensity 
may vary, it is evident that the results at Rock Rapid, Point Beaufort, and Montreal 
Island, in either of these Tables, cannot be in accordance with those at Fort Re- 
liance, Musk-Ox Rapid, and Point Ogle ; and it may be worth inquiring to what this 
discordance, and that between these results and those with the dipping needle No. II., 
are to be attributed. That there may have been errors in the observations of the 
times of vibration of the needles is very possible, but not to the amount that such 
discordances would indicate ; and I therefore attribute the want of agreement in the 
results to the inefficiency of Hansteen’s method for the determination of the absolute 
intensity in such cases, rather than to errors of this description. When the dip is 
great, a small error in its determination will introduce a large one in the determina- 
tion of the absolute intensity from the horizontal ; and to such errors I consider that 
these discordances may, in a great measure, be attributed. In order to determine the 
errors in the dip necessary to account for this want of agreement between the results 
obtained with the dipping needle No. II. and with the horizontal needles, let i be the 
measure of the horizontal intensity, and c> the dip at the place of observation, I the 
measure of the absolute intensity at London with the same needle, and M the ratio 
of the intensity at the place of observation to that at London, as determined by the 
time of vibration of the needle No. II., then 
cos £ = (22.) 
In the following Table are given the values of c> deduced from this formula by 
substituting for M its values in the Table at p. 408, and for I its values in each of 
the two foregoing Tables : 
3 g 2 
