MADE BY CAPTAIN BACK DURING HIS LATE ARCTIC EXPEDITION. 
381 
that I, by any means, consider that the dip has been in this, or that it is in any case 
determined to anything like such a degree of accuracy; but because such results ren- 
dered testing the accuracy of calculation more convenient ; and that having, in sub- 
sequent calculations, employed the actual results obtained, I have thought it right to 
give them in all cases. 
In order to show the care which Captain Back took, in making the observations, to 
be as free as possible from the effects of any particular local influence, it is necessary 
that I should give his notes on the situations, with reference to surrounding objects, 
of the spots on which the observations were made ; and it will be proper that I should 
do so previous to making any remarks on the results contained in the foregoing Table. 
New York. “ The observations were made in the garden of the British consul under 
a temporary shed, erected for the purpose, and distant from the house eighty paces.” 
Montreal. “ The observations were made under a tent on the island of St. Helen’s, 
in the St. Lawrence, one thousand yards from the city of Montreal. There was no 
iron near; the roofs of the houses in the city are most of them covered with tin.” 
Fort Alexander'. There is no note appended to the observations, on the position of 
the instrument here; but, in his Narrative, Captain Back makes a remark on some 
discrepancies in the dip of the needle, which he attributes to the influence of a distant 
thunder storm*. 
Cumberland House. “The observations were made in a tent about two hundred 
yards from the house.” 
Isle a la Crosse. “The observations were made in a tent forty paces from the 
stockades of the fort.” 
Fort Chipewyan. “ The observations were made in a tent to the westward of the 
fort.” 
Fort Resolution. “ This set was made in a tent placed inside of the stockades of 
the fort, but quite free from the influence of any iron.” 
It is necessary I should remark, that in this set of observations the differences in 
the observed angles were considerable, with the face of the needle reversed and the 
face of the instrument west. Six observations were made in this position ; they were 
as follow : 79° 00' ; 79° 17'*5 ; 79° 55' ; 80° 5' ; 80° 12' 5 ; 80° 1 7' 5. The result given 
in the eighth column of the Table is the mean of the last four : I rejected the first two, 
because the others are very accordant and indicate no disposition in the needie to 
return to its first position. On the subject of these observations Captain Back has 
made this remark: “At the time the needle changed from 79° 15' to 80° (lower 
readings) the weather was more than commonly gloomy, and some few drops of rain 
fell.” During the time of making this whole set of observations “ the weather was 
gloomy, dark, and overcast, with light rain at intervals :” so that the discrepancy 
here noticed was most probably owing to some change in the electric state of the 
atmosphere. 
* Narrative of the Arctic Land Expedition, p. 41. 
