Terrestrial Magnetism. 321 



stood. There is certainly no better authority for the dip at any 

 particular time and place, than careful observations made with 

 the best instruments. When artificial lines do not agree with 

 observations, it is evident that those lines should be marked "m 

 errory The lines of equal dip, as obtained by actual survey, 

 are not great circles nor uniform curves ; they undulate irregu- 

 larly, converging in some places and diverging in others, and 

 sometimes, I believe, one line of equal dip will divide into two 

 which will afterwards reunite. It is perhaps customary with 

 those who make magnetical charts, to endeavor to equalize these 

 natural irregularities, as the engineer after a survey for a road 

 equalizes the hills and hollows to obtain a less devious but more 

 artificial surface. Such a line, although it is easily projected, and 

 looks well in the chart, has no existence in nature, and is only 

 an artificial approximation to truth ; so far as it departs from the 

 results of actual observations, the line itself should be marked in 

 error, not the observations. This, I say, would be philosophical; 

 a conventional mode of expressing the same relation in different 

 terms may obtain a preference, and would be unobjectionable pro- 

 vided it should be properly understood. 



The largest number of my magnetical experiments were made 

 in Iowa and Wisconsin Territories during last autumn, and the 

 general results, including both the dip and intensity, have been 

 communicated both to Congress and to the American Philosophical 

 Society. But I am reminded by the above suggestions that I 

 ought to lay before the public a specimen of the details at large, 

 that a proper judgment may be formed of the degree of credit to 

 which they are entitled. I shall confine myself at present to that 

 part of my observations for determining the dip. My dipping 

 compass was made by Robinson, of London, in 1837. As a 

 check upon errors, I make at each station, by means of two sep- 

 arate needles, a double suite of observations. In each suite, all 

 of the usual reversals are made, including the face of the instru- 

 ment, the face of the needle, and the polarity of the needle by 

 retouching. The dip is therefore determined by eight distinct 

 readings of each needle, the two results almost always agreeing 

 within one or two minutes of a degree. The mean of the whole 

 of the sixteen readings is finally taken. The following are ex- 

 amples : 



