88 Magnetic Dip in the United States. 
lines is from N. 65° 1’ W. to S. 65° 1’ E. Computing the dip 
from these data we obtain the differences in the last column 
above. These differences are much less than those before found, 
and it seems highly probable that in this vicinity the isoclinal 
lines make a greater angle with the parallels of latitude than they 
do in Ohio. Yet the above observations are all embraced within 
less than two degrees of longitude, and are therefore insufficient 
to determine with much precision the dependence of the dip upon 
the longitude. I think it improbable that the inclination should 
be as great as 24° 59’ according to these observations; yet ad- 
mitting such to be the case, we still obtain considerable differen- 
ces between the observed and computed dip. Are these differ- 
ences to be regarded as errors of observation, or as errors of the 
hypothesis of parallel, straight and equidistant isoclinal lines? 
In order to answer this question, it is necessary to consider all 
the possible sources of error in magnetic observations. 
The errors arising from the inclination of the magnetic axis of | 
the needle to the axis of figure, and from inequality in the weight 
of the arms, as well as the zero error of the graduation, appeat 
to have been provided against in Prof. Locke’s mode of observa 
tion. That arising from the excentricity of the axis of the nee 
dle in relation to the vertical circle on which the readings ate 
made, is not alluded to. This error in my instrament commonly 
amounts to one or two minutes, and sometimes even to five oF 
more. It is corrected by readings at both extremities of the nee 
dle. Prof. Locke makes no mention of having employed this 
_ precaution, and his language on page 321, where he says “the 
dip is determined by eight distinct readings of each needle,” 
would seem to imply that he did not attend to it. With a good 
instrument, no great error would ordinarily arise from this source, 
yet it might easily amount to 2’ or 3’. 
A more considerable source of error is that arising from the 
uncertainty of the readings themselves. <A dipping needle will 
seldom come to rest twice in the same position. ‘This arises, no 
from a change in the direction of the magnetic force, but from 
friction on the axis of the needle. Prof. Locke’s observations 
exhibit this fact in a striking light. The difference of the read 
ings with the face of the instrument east and west, and in the 
same position of the needle, is equal to twice the zero error. Now 
as this error may be assumed to be constant, we obtain by a cour 
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