MADE BY CAPTAIN BACK DURING HIS LATE ARCTIC EXPEDITION. 405 
Thermo- 
meter 1. 
Differ- 
ences. 
Thermo- 
meter N. 
Values of — . 
M 
Differences. 
Difference in 
value of — 
M 
corresponding to 
difference of 1° 
in Therm. 1. 
46-2 
o 
60-0 
•4664903 
78-0 
31-8 
61-4 
•4549698 
•0115205 
•0003623 
47-5 
30*5 
60-8 
•4641094 
•0091396 
•0002997 
Mean 
•0003310 
Again, making use of similar methods of reduction with these results, the equation 
for the correction of the observations made with the needle No. 1 becomes 
I 
h 
1 + -0007 181 6 
(17. l.) 
Reduction of the Observations. 
Having pointed out the methods which I adopted for determining the constants 
which enter into the equations requisite for the reduction of observations, made with 
these needles at different temperatures, to results at a standard temperature, and 
given the equations necessary for such reduction, I now resume the consideration of 
Captain Back’s observations for the determination of the relative terrestrial magnetic 
intensities at different stations. 
I propose first to determine these intensities from the observed times of vibration 
of the dipping needle No. II. in the plane of the meridian, and then to inquire whether 
the results derived from the times of vibration of the horizontal needles are in ac- 
cordance with these. 
In order to determine the intensities from the times of vibration in the meridian, 
it is necessary to divide the observations into two classes ; first, those from London 
to Fort Reliance ; and secondly, those from Fort Reliance to Point Ogle, on the coast 
of the Polar Sea, and thence to London. 
Taking the reciprocal of the square of the time of vibration as the measure of the 
intensity at the temperature at which an observation was made, this measure is to be 
reduced by means of the formula (17- II.), in order to determine the measure of the 
intensity at a standard temperature. If t is the time of vibration of the needle at the 
temperature 60° + 6, and I the intensity at the standard temperature 60°, then 
* = t 2 {\ + *000454 0 ) (18.11.) 
Taking the times of vibration of the needle No. II. and the temperatures already 
given, the corresponding measures of the intensity at the different stations, as de- 
duced from this equation, are given in the following Table, and likewise the relative 
intensities, that at London being taken as unity. In deducing the latter, two kinds 
