294 
MAGNETIC SURVEY IN NORTH AMERICA. 
are brought together for the purpose of showing by the agreement in the resulting 
values of X, the general accuracy of the constants, whether the bar was vibrated 
in the stirrup, or without it. 
Table XXXVII. 
Date. 
Station. 
No. 30. 
No. 31. 
Values of X. 
Diff. 
AX. 
AX 
X ‘ 
Values of X. 
Diff. 
AX. 
AX 
X ‘ 
With 
stirrup. 
Without 
stirrup. 
With 
stirrup. 
Without 
stirrup. 
1843. 
August 16. 
Cross Lake .... 
2-348 
2-361 
+ •013 
+ •006 
2-348 
2-349 
+ •001 
+ •001 
23. 
Cumberland. . . . 
2-345 
2-345 
•000 
•000 
2-337 
2-337 
•000 
•000 
September 9. 
Isle a la Crosse. . 
2-387 
2-397 
+ •010 
+ •004 
2-387 
2-386 
—•001 
•000 
October 12. 
Athabasca .... 
2-021 
2-025 
+ •004 
+ •002 
2-027 
2-030 
+ •003 
+ •001 
1844. 
March 2. 
Athabasca .... 
2-018 
2-025 
+ •007 
+ •003 
2-020 
2-008 
-•012 
-•006 
May 2. 
Fort Simpson . . 
1-936 
1-961 
+ •025 
+ •013 
1-946 
1-966 
+ •020 
+ •011 
29. 
Fort Good Hope. 
1-666 
1-675 
+ •009 
+ •005 
1-680 
1-680 
•000 
•000 
1845. 
Jan. and Feb. 
Toronto 
3-528 
3-522 
-•006 
— •002 
3-539 
3-539 
•000 
•000 
September . . 
Toronto 
3-510 
3-511 
-•001 
•000 
3-514 
3-518 
•004 
-•001 
“ If we omit the observations at Fort Simpson, at which the difference between the 
values of X by the two methods of suspension is considerable with both bars, and 
must have been due to some other cause than a discrepancy between the constants, 
the mean difference in the values of X, irrespective of signs, is for 
No. 30. *0029= of X, 
No. 31. '0012=_l_ o fx 
833 
“ Experiments of Vibration . — Each time of vibration has been corrected for the rate 
of the chronometer, which was generally large, 10 s to 20 s ; and as the chronometer 
was usually worn in the pocket, under circumstances not favourable to its perform- 
ance, the actual rate on any given day may have sometimes differed to a sensible 
amount from the mean rate applied. The arc of vibration was not recorded ; but as 
a general rule it was reduced at the commencement to an apparent value of one 
diameter of the end of the magnet as seen through the theodolite, which is equiva- 
lent to 12°. As the magnet carried no reflector, and there were no direct means of 
measuring the exact amount of the arc, an approximate correction has been applied, 
by assuming the mean value of the initial semi-arc 360', and of the terminal semi-arc as 
|T 
200'. The same circumstances prevented any direct measurement of the value of 
but a mean value for each mode of suspension employed was ascertained at Toronto, 
by trying several suspension threads of the same number of fibres, and of the same 
silk, as were employed on the Survey. 
