MADE BY CAPTAIN BACK DURING HIS LATE ARCTIC EXPEDITION. 409 
must lead to the same conclusions, I have considered it would be satisfactory that 
such a comparison should be made. A most striking agreement is here manifest 
between the computed values of I and those deduced from observation, in every in- 
stance from New York to Point Ogle. It will be seen that the greatest error in the 
computed value of I, that corresponding to the observations at Fort Alexander, does 
not amount to a forty-fifth of the intensity ; and if a mean of all the errors, without 
regard to their signs, be taken, it will be found that this mean error does not amount 
to the one hundred and fourth part of the mean of all the intensities. Such differ- 
ences as are here indicated are quite within the limits of the errors to which observa- 
tions of this nature, when carefully made, are liable, and are much less than are, 
generally, to be expected in any comparison of results with a formula that ought to 
represent them, when those results are deduced from the vibrations of a needle. It 
certainly does appear that the intensity did not in all cases increase with the dip ; but 
it is probable that these discrepancies have arisen either from small errors in the ob- 
servations, or from local causes affecting the intensity of the terrestrial force. Upon 
the whole, I think we are fully warranted in concluding that in the track of country 
embraced by Captain Back’s observations, from New York to the Arctic Sea, the phe- 
nomena of terrestrial magnetic intensity are very correctly represented by the for- 
mula with which I have compared them. 
I now proceed to discuss the observations made with horizontal needles for deter- 
mining the intensity. 
In order to determine the measures of the horizontal intensity resulting from the 
observed times of vibration of the three needles given in a preceding Table (p. 397), 
it is necessary to apply corrections for the difference of temperature, as indicated in 
the equations (17- 1.)* (17- 3.), (17- L.) ; and in the following Table I have given the 
results so corrected : 
Place of observation. 
Needle No. 1. 
Needle No. 3. 
Lozenge-needle. 
Time of 
one 
vibration. 
Therm. 
Measure of hori- 
zontal intensity 
at Temp. 60°. 
Time of 
one 
vibration. 
Therm. 
Measure of hori- 
zontal intensity 
at Temp. 60°. 
Time of 
one 
vibration. 
Therm. 
Measure of hori- 
zontal intensity 
at Temp. 60°. 
London, Feb. 7, 1833 
4-445 
56-0 
•0504675 
4*36167 
56-0 
•0524378 
1-8525 
50-0 
•290366 
Montreal 
No observations made. 
4-82857 
56-5 
•0428001 
No observations made. 
V 
f Oct. 11, 1833. . 
7-7000 
45-5 
•0166925 
7-4790 
46-0 
•0177276 
3-1535 
46-0 
•1000563 
u § 
Feb. 8,1834.. 
No observations made. 
7-4941 
45-25 
•0176483 
3-14844 
35-0 
•0999945 
May21, 1834. . 
o 
pH 
to 
CO 
0*5 
00 
<£> 
•0168047 
7-42692 
50-5 
•0180258 
3-1650 
51-0 
•0995105 
Pd 
Oct. 9,1834.. 
No observations made. 
7-6281 
30-0 
•0168793 
3-1423 
30-0 
•1002096 
Musk-Ox Rapid. . . . 
No observations made. 
8-8909 
7i-o 
•0127353 
No observations made. 
Rock Rapid 
No observations made. 
13-0286 
82-0 
•0059707 
No observations made. 
Point Beaufort .... 
No observations made. 
18-5000 
74-0 
■0029468 
No observations made. 
Montreal Island .... 
No observations made. 
16-7750 
67-5 
•00356986 
6-880 
63-5 
•0211526 
Point Ogle 
No observations made. 
22-7125 
62-13 
•00194097 
8-825 ' 
63-5 
•0128561 
London, Feb. 12, 1836 
4-6632| 45-0 
•0455006 
4-5588 
45-25 
•04769145 
1-8944 
44-75 
•277152 
If we compare the measure of the intensity in London previous to Captain Back’s 
departure with its measure by the same needle on his return, it is evident that each 
MDCCCXXXVI. 3 G 
