Magneticdl Dip in the United States. 17 
in the two groups, have bearings differing 19° 53/ from each 
other. What will be the bearings of those lines in the interme- 
diate region? Ought the two lines to be produced, the one from 
Jowa, and the other from Ohio, until they meet? The two 
would not certainly form one “ straight line,’ which is called for 
by the hypothesis. Or, ought there to be an intermediate “ cen- 
tral position adopted,” and a new bearing of the isoclinal line de- 
termined? 'This would likely give us a polygon. Now “ adopt,” 
or assume an infinite number of “central | positions,” and: caleu- 
late as many bearings of the isoclinal or equal dip lines, and the 
result will be curves, precisely what I contend for. But thisis _ 
contrary to the “ hypothesis,” which calls for “straight lines.” 
Here might bea very great modification of the “ computed errors 
of observation.” My own opinion, founded on observations of 
considerable extent, is that the lines do proceed in curves which, 
in the present state of our knowledge, can be traced only by ob- 
servation. ‘The hypothesis that the magnetical forces proceed in 
es, less tortuous in their course than observations would seem 
to indicate, is one by no means of recent date ; for it is found in 
the philosophical writings of the Baron Suedenbots, together 
with a very specific account of the a in ‘making 
observations. ‘The appearance of mat ala : 
cially under the symbols of modern ¢ 7 Sep inipshing ¢ 
many a popular reader, who may not refi t all mathemati 
results are dependent on the data, the assumptions, 
ses; if these be false or erroneous, the superstructure is but a 
wreath of mist. Grant the data, and the calculations in t 
times, are but a schoolboy’s task.* 
On page 87, Prof. Loomis observes in - iehiahlne to my oben 
vations, as treated in his previous paper, “ at Prairie du Chien, the 
discordance is more considerable. The difference [from the hy- 
pothesis] I now find is —7’.3 ; in my former paper —20’. The 
discordance is owing in part to the curvature I ascribed to the 
isoclinal lines, by which most of the observations seemed best 
represented, though the apparent error of this observation was 
increased.” All this is perfectly candid. He admits a curvature’ 
in the “straight lines,” and that the lines are those of his own 
creation, subject to be changed at pleasure ; not the isoclinal 
* It is evident from the manner in which Prof. L. uses the hypothetical straight lines, 
that they were not really the lines of equal dip, Se ae eee those lines. 
Vol. xu1, No. 1 —April-June, ao * 
Dog 
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