MADE BY CAPTAIN BACK DURING HIS LATE ARCTIC EXPEDITION. 401 
indications of the thermometer N were not constant, but, as the whole difference did 
not amount to 3°, I do not consider that this can materially affect the result. It is, 
however, probable that the difference in the temperature of the needle N did not 
even amount to this : for in consequence of the glass shade under which the needle 
vibrated being too small to contain a thermometer, that instrument was placed on 
the outside, and would therefore be more affected by small changes in the tempera- 
ture of the room than the needle which was inclosed. 
771 
If we take the mean of the values of ^ corresponding to the indications 59°’8 and 
771 
60°’3 of the thermometer II., we shall have ’7742069 as the value of ^corresponding 
to the temperature 60°’05 of the needle II. Adding ’0000175 to this value, for the 
771 
difference 0°’05, we have ’77 42244 for the value of ^ at the temperature 60°. If now 
771 
we consider the intensity of the needle II., or the value of ^ at 60° Fahr. to be 1, 
we shall have. 
m 
The increment in the value of ^ for each 1° above or below 60° 
+ -00035 1 5 
"’7742244 
= If ’0004540. 
Let I / be the measure of the terrestrial magnetic intensity resulting from observa- 
tion, at any station, of the time of vibration of the needle No. II., at the temperature 
60° + 6 ; and I the measure of the intensity at the temperature 60° : then 
I, = I Ip ’000454 Id ; 
and 
* = 1 + -000454 0 U-) 
By means of this formula, the measure of the intensity with the needle II. at any 
temperature 60° + 0 may be reduced to the measure of the intensity at the standard 
temperature 60° Fahr. 
In order to determine the corrections to be applied, for difference of temperature, to 
the observations made with the horizontal needles No. 1, No. 3, and the Lozenge- 
needle, experiments similar to those which I have described were made with these 
needles. As these experiments were made consecutively, it is necessary that I should 
give them in the order in which they were made. 
With the Lozenge-needle, two sets of experiments were made. In the first set, 
this needle occupied the same position precisely as that in which the needle No. II. 
had previously been placed. The results are given in the following Table : 
3 F 
mdcccxxxvi. 
