2 OBSERVATIONS TO DETERMINE THE MAGNETIC DIP 



The observations were invariably made in an open area, at the distance of 

 several rods from any building, or any apparent local cause of attraction. Par- 

 ticular care was taken to remove all iron in the form of knives, keys, &c. 

 The instrument was placed upon a solid block of wood of convenient height, 

 and levelled. The vertical circle was then turned in azimuth until the needle 

 assumed a vertical position,- and the azimuth read off from the horizontal circle. 

 The needle was then turned upon its supports, (the north extremity of the axis 

 to the south,) and the observation repeated. Needle No. 1, in which the dis- 

 tribution of the magnetism was most uniform, was always used for this pur- 

 pose, and the two readings ordinarily differed by less than a degree. The 

 mean of the two was taken as indicating the vertical plane at right angles 

 to the magnetic meridian. In order to test the degree of accuracy of which 

 the method is susceptible, I made repeated observations at Hudson, where I 

 had a meridian mark, and knew the variation of the needle. The preceding 

 method was found to give the magnetic meridian within a fraction of a degree. 

 Let us inquire what influence such an error would produce upon the observed 

 dip. 



Put S = the dip in the magnetic meridian. 

 = the dip in any vertical plane. 



A = the magnetic azimuth of the plane in which S' is observed. Then 

 we shall have tang b' = tang S. sec. A.; from which formula we learn that 

 at Hudson the dip increases less than one minute, from being observed two 

 degrees out of the magnetic meridian. The method employed for determining 

 the magnetic meridian possesses, therefore, all the accuracy which could be 

 desired for this purpose. 



Both needles were observed at each station, and an equal number of times. 

 In needle No. 1 the magnetic axis was found always to coincide very nearly 

 with the geometrical axis. Although I have reversed the poles more than a 

 dozen times, the inclination of the magnetic axis to the geometrical has never 

 exceeded a small fraction of a degree. In needle No. 2, although it has been 

 magnetized in the same way, and its poles reversed the same number of times, 

 the magnetic axis has invariably been found quite oblique to the geometrical 

 axis, the inclination varying from one to three degrees. For reversing the 

 poles I always employ a bar magnet of about a foot in length. I draw the fiat 

 side of one half the needle over a pole of the magnet; then the opposite side 



