326 Terrestrial Magnetism. 
one plus and the other minus, as it would be merged by both be- 
ing plus or both minus; and hence half of the greatest difference 
by the two needles may be taken as the limit of instrumental 
errors, which in the above observations would be only 0/.862 
a minute; a quantity much smaller than I should have anticipa- 
ted before making the examination. The instrument is evidently 
a very perfect one, yet at certain points, when the dip arrives at 
a particular quantity, probably from a want of perfect roundness 
of the pivots, one needle will read constantly and uniformly more 
than the other. Thus at Dubuque, where the dip is 73° 04’, 
needle No. 1, read in the mean, constantly 24’ more than No. 2. 
Even this error would cidiparily be considered very small. The 
French have a saying that “the dipping compass is one very un- 
grateful instrument.” But with this fine piece of workmanship 
of Robinson’s, I have repeatedly admired the beautiful manner 
in which the reversals correct all of the errors, and the two nee- 
dies, none of whose individual readings are alike, will ultimately 
in the mean, give almost identical results. 
In my surveys, I did not expect the dip to be so little at Prairie 
du Chien, nor so great at Madison, Wisconsin Territory, as I 
found it. I went through with four suits of observations at the 
former place, before I was satisfied of their correctness. But all 
of the observations between Prairie du Chien and the four lakes, 
_ agree in determining that the lines of equal dip along the Wis- 
consin river, in advancing westward, incline rapidly to the north. 
From a point about five miles south e Mineral Point, the line of 
dip for 73° 16’ passes to Prairie du Chien, in the direction of 
west 22° 10’ north. The curvature between Madison and Blue 
Mounds, is probably still more to the north. At Dubuque, how- 
ever, there was no evidence of such a northern inclination of the 
lines of equal dip. At Columbus, Ohio, the dip appeared to be 
so much less than I expected, that after determining it twice at 
our station, suspecting local attraction, I removed to another a 
mile distant, but the result, was still the same. At the very points 
marked by Prof. Loomis, I anticipated the departure of the mag- 
netical quantities from their general direction, and was especially 
Mawtions in my examinations, but finally was compelled to record 
results in obedience to the authority of nature. I believe 
are so many anomalies in the elements of terrestrial magnet- 
ism, that the only. aie way in parcconting with, our surveys, is to 
