MADE EY CAPTAIN BACK DURING HIS LATE ARCTIC EXPEDITION. 407 
and first navigator, should, henceforward, be named the Back , — the needle was, in 
consequence of other most pressing calls on his time, only vibrated with its face to the 
face of the instrument, excepting at Point Ogle, where a vexatious detention afforded 
more time for these observations. In comparing, therefore, the results from these 
observations with those at Fort Reliance, I have only taken those derived from ob- 
servations made there in the same position of the needle. I have, however, deduced 
the measure of the intensity at Point Ogle both from the observations made in this 
position of the needle, and also from a mean of these and of those with the needle 
reversed on its axis. In the following Table are given: — the measures of the intensities 
at the temperature 60° ; the relative intensities, taking the mean of the intensity at 
Fort Reliance before quitting, and on the return to that station, as unity ; and like- 
wise the relative intensities, taking that at London, on Captain Back’s return, as 
unity. 
Place of observation. 
Date. 
Time of 
vibration of 
needle. 
Thermo- 
meter. 
Measure of in- 
tensity at 
temp. 60°. 
Ratio of in- 
tensity to that at 
Fort Reliance. 
Ratio of in- J 
tensity to that j 
at London. 1 
Fort Reliance <j^ 
Musk-Ox Rapid .... 
Rock Rapid 
Point. Beaufort 
1834. 
May 21. 
Oct. 8- 
July 2. 
July 23. 
July 31. 
Aug. 2. 
Aug. 12. 
1836. 
Feb. 12. 
s 
1-28440 
1-27813 
Mean . . . 
49-13 
27-62 
•603192 
•603416 
•603304 
•604584 
•617855 
•597388 
•606212 
•622317 
•480910 
1-00000 
1.00212 
1-02412 
0- 99020 
1- 00482 
1-03152 
0-79713 
1-25450 
1-25717 
1-28473 
1-24220 
1-26055 
1-29404 
1-00000 
1-26576 
1-28726 
1-28000 
1-2975 
1-2885 
1-26563 
1-4368 
64-0 
86-75 
72-5 
74-12 
53-0 
44-0 
Montreal Island .... 
Point Ogle 
London 
Fort Reliance 
May 21.1 
Oct. 9- J 
Aug. 12. 
Mean{^ d 
1-27507 
le direct 1 
reversed J 
54-06 
•608175 
•613426 
1-00000 
1-00863 
Point Ogle 
On the hypothesis of two magnetic poles not far removed from the centre of the 
earth, to which I have already adverted, if I represent the intensity of the force in 
the direction of the dip, then 
^ C (3 cos 2 S + 5) ‘ ( 19 ‘) 5 
where p is a constant, whose value may be assumed in a comparison of the inten- 
sities corresponding to different values of £, or may be determined from the observed 
values of I. 
If we assume the intensity at the magnetic equator to be 1, then ;jj = 2 V2, and 
the equation becomes 
* = */ (3 cos 2 8 + 5) 
In the Table which follows I have given : — the dip as already determined from Cap- 
tain Back’s observations ; the intensities as deduced in the preceding Tables, taking 
I 
