MADE BY CAPTAIN BACK DURING HIS LATE ARCTIC EXPEDITION. 
395 
to l s, 554 ; but even this is a most improbable result. In the subjoined Table I give 
the times of beginning and ending the vibrations, with Captain Back’s remarks on 
the weather, &c. at the times of observing. 
Place of 
obser- 
vation. 
Face of the needle to the face of the instrument. 
Face of the needle reversed. 
Face of the instrument 
Face of the instrument 
East. 
West. 
East. 
West. 
Cumberland 
House. 
Time commencing 10 h 22 m 25 s . 
Time concluding 10 24 40. 
Wind S.W. by compass. 
Weather squally, with rain ; 
cloudy. 
Time commencing 10 h 50 m 22 s . 
Time concluding 10 52 30. 
Wind S.W. by compass. 
Weather squally, with light 
rain; overcast. 
Time commencing 9 h 47 m 32 s . 
Time concluding 9 50 41. 
Wind S.W. by compass. 
Weather squally, with rain ; dark 
clouds, but no thunder. 
Time commencing 9 h 18 m 59 s . 
Time concluding 9 21 31. 
Wind S.W. by compass. 
Weather calm and sultry, with 
distant thunder. 
Isle a la 
Crosse. 
Time commencing 8 h 37 m 00 s . 
Time concluding 8 38 23. 
Weather overcast, calm. 
Needle sluggish. 
Time commencing 8 h 56 ra 30 s . 
Time concluding 8 59 00. 
Weather clearer ; the sun seen 
at intervals. 
Time commencing 9 h 50 m 30 s . 
Time concluding 9 58 25. 
Weather clearer ; sun out. 
Time commencing 9 h 28 m 00 s . 
Time concluding 9 29 39. 
Weather clearer. 
The needle was decidedly sluggish though carefully wiped. 
At Rock Rapid it appears that there was considerable difficulty in counting more 
than forty or fifty vibrations of the needle, its motion being extremely unsteady, and 
occasionally it came to a dead stop. I have selected the two sets of vibrations in 
which it continued vibrating for the longest time. In two other sets, in similar posi- 
tions, it made only thirty-two vibrations in the one case, and thirty-eight in the other: 
Captain Back has a remark on these observations in his Narrative*. It is difficult 
to conceive any local cause for this tendency of the needle to come so much quicker 
to rest than in other cases, and I cannot but attribute it to the influence of the sun-i~. 
The observations were made as usual in a tent, but the height of the thermometer 
clearly indicates that the sun must have had considerable influence even under this 
screen. This view of the cause of the needle so soon coming to rest at Rock Rapid, 
is corroborated by the fact that, at Montreal Island “ a sail was put over the tent to 
afford more shade,” and there “ the needle was particularly lively, and vibrated 
smoothly until it finally rested,” the number of vibrations being one hundred. What- 
ever may have been the cause which thus affected the vibration of the needle, the 
circumstance must throw some uncertainty on the measure of the intensity deduced 
from its time of vibration. 
Although I have found it necessary to make these remarks on Captain Back’s ob- 
servations, it cannot, I trust, be supposed that I attribute the errors which I have 
pointed out to want of care or attention on the part of that most enterprising officer, 
for no one can appreciate more highly than I do the zeal which he manifested for the 
promotion of scientific research, by undertaking to make these observations on such 
an expedition as that on which he so nobly volunteered. I attribute these errors 
solely to Captain Back’s destitution of assistance ; and I cannot but regret that it 
* Narrative of the Arctic Land Expedition, p. 360. 
f Philosophical Transactions, 1826, p. 219; 1828, p. 379. 
3 E 2 
