ARTICLE XIX 



Observations of the Magnetic Dip, made in the United States, in 1841. By 

 J. N. Nicollet. Read September 16, 1842. 



The following observations were made with a dipping instrument con- 

 structed by Robinson, of London, and procured for me by Professor A. D. 

 Bache of Philadelphia. The whole instrument is made of brass; but the 

 vertical circle is plated with silver, upon which the dip is read directly to ten 

 minutes, which, by estimation, may be easily divided to one or two minutes. 

 The horizontal circle, which serves for the measurement of the azirauthal 

 motions of the vertical plane, is graduated to degrees, and the position of the 

 magnetic meridian is determined, either by the usual method of measuring 

 the position of the plane perpendicular to that meridian; or, a horizontal 

 needle mounted upon a pivot with a cylindrical handle of brass having an axis, 

 may be used instead of the dipping needle, being placed in the magnetic me- 

 ridian, and by the aid of the azimuthal motion of the vertical circle, is found 

 to answer the purpose even better than the other mode. The two dipping 

 needles which accompany the instrument are each of them six inches in 

 length, and in the form of rhombs, terminated very acutely towards each ex- 

 tremity. The breadth across the shorter diagonal of the rhombs is four-tenths 

 of an inch. The axis, when the observation of the dip is made, rests upon 

 agate supports, and its uniform central position upon them is secured by two 

 brass Y's, which can be gently raised and lowered at will, so as to relieve the 

 needle from the supports, or restore it to its bearing upon them, and affords 

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