OBSERVATIONS OF THE MAGNETIC FORCE. 
309 
Determination of the absolute Horizontal Force derived from the times of vibration 
of the magnets of the portable unifilar magnetometer in 1843. 
Besides the ten stations at which observations both of deflection and of vibration 
were made with Nos. 30 and 31 in 1843, which have been discussed in pp. 289-302, 
there were also thirteen stations at which the times of vibration only were observed, 
unaccompanied by observations of deflection : these stations were all comprised be- 
tween Cumberland House on the 22nd of August and Athabasca on the 1 3th of October. 
The deflections observed at Cross Lake and Cumberland House on the 16th and 22nd 
of August, and at Athabasca on the 13th of October, indicate that the magnetic 
moment of both magnets sustained a considerable diminution in the interval ; and 
from the only intermediate determination, made at Isle a la Crosse on the 9th of Sep- 
tember, we may infer that the greater part at least, if not the whole of the loss, 
occurred in both magnets between August 23 and September 9. For the second por- 
tion of the interval therefore, or from September 9 to October 23, the arithmetical 
mean of the determinations at Isle a la Crosse and at Athabasca have been taken for 
the values of m, viz. *4138 for No. 30, and ’3841 for No. 31. In regard to the pre- 
vious interval, or that comprised between August 22 and September 9, the observa- 
tions at Cumberland House and at Isle a la Crosse manifest that a very considerable 
change took place in the magnetic moment of both bars, but as intermediate obser- 
vations of deflection are wholly wanting we have no direct evidence of the particular 
time when it occurred. A careful examination and comparison of the times of vibra- 
tion at the intermediate stations makes it probable that the greater portion at least 
of the loss occurred in No. 31 between Cumberland House and Beaver Lake, and in 
No. 30 between Beaver Lake and the Portage des Epinettes ; but as there is neces- 
sarily much uncertainty involved in any conclusion on this point, it has appeared the 
safest course to record the times of vibration at the stations between Cumberland 
House and Isle a la Crosse without an attempt to deduce the horizontal force from 
them. 
The times of vibration of No. 17 were also observed at the stations between Cum- 
berland House and Athabasca, but no deflection experiments were made with it 
before the arrival at Athabasca; it appears by the observations of vibration that this 
bar unquestionably lost magnetism in the interval, but as there is no independent 
evidence to show at what particular time the loss took place, or whether it was of a 
sudden or of a progressive character, the times of vibration with this magnet have 
also been recorded, but no conclusion has been drawn from them in regard to the 
values of the horizontal force at any earlier station than at Athabasca. 
2 s 
MDCCCXLVI. 
